feat(llm): halkyon_llm, language models' APIs streamed over plain HTTP (ADR 0021, step 2)

A pure Dart package, no SDKs, one interface (LlmClient) over Anthropic's
Messages API, OpenAI's chat completions, in each known host's dialect and
the local servers' (D45), and OpenAI's Responses API, stateless (D46). A
model's turn keeps what its provider wants back with the tools' results:
Anthropic's thinking blocks, the Responses API's reasoning items and
message phase, Gemini's thought signatures. Failures come by kind, the
provider's words cut and without keys.

Tested on replies written from the providers' documentation, replayed in
pieces of 1, 7 and 4,096 bytes; tool/record_llm.dart records real ones
with a key of one's own, which the tests then run on too (D36).
FakeLlmClient for the code that uses it. BSD-3-Clause, as halkyon_oidc.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

packages/halkyon_llm/test/fixtures/documented/anthropic/overloaded_midstream.http

@@ -0,0 +1,15 @@
+HTTP 200
+content-type: text/event-stream; charset=utf-8
+
+event: message_start
+data: {"type":"message_start","message":{"id":"msg_01Qm2vYB8xkN4cH6sGtRpL3J","type":"message","role":"assistant","model":"claude-sonnet-5-5","content":[],"stop_reason":null,"stop_sequence":null,"usage":{"input_tokens":412,"output_tokens":1}}}
+
+event: content_block_start
+data: {"type":"content_block_start","index":0,"content_block":{"type":"text","text":""}}
+
+event: content_block_delta
+data: {"type":"content_block_delta","index":0,"delta":{"type":"text_delta","text":"„Hund“"}}
+
+event: error
+data: {"type": "error", "error": {"type": "overloaded_error", "message": "Overloaded"}}
+

packages/halkyon_llm/test/fixtures/documented/anthropic/status_400.http

@@ -0,0 +1,4 @@
+HTTP 400
+content-type: application/json
+
+{"type":"error","error":{"type":"invalid_request_error","message":"model: claude-sonet-5-5 is not a valid model"},"request_id":"req_011CSHoEeqs5C35K2UUqR7G0"}

packages/halkyon_llm/test/fixtures/documented/anthropic/status_401.http

@@ -0,0 +1,5 @@
+HTTP 401
+content-type: application/json
+request-id: req_018EeWyXxfu5pfWkrYcMdjWG
+
+{"type":"error","error":{"type":"authentication_error","message":"invalid x-api-key"},"request_id":"req_018EeWyXxfu5pfWkrYcMdjWG"}

packages/halkyon_llm/test/fixtures/documented/anthropic/status_402.http

@@ -0,0 +1,4 @@
+HTTP 402
+content-type: application/json
+
+{"type":"error","error":{"type":"billing_error","message":"Your credit balance is too low to access the Claude API."},"request_id":"req_011CSHoEeqs5C35K2UUqR7G1"}

packages/halkyon_llm/test/fixtures/documented/anthropic/status_429.http

@@ -0,0 +1,5 @@
+HTTP 429
+content-type: application/json
+retry-after: 30
+
+{"type":"error","error":{"type":"rate_limit_error","message":"This request would exceed the rate limit for your organization of 50 requests per minute."},"request_id":"req_011CSHoEeqs5C35K2UUqR7Fy"}

packages/halkyon_llm/test/fixtures/documented/anthropic/status_529.http

@@ -0,0 +1,4 @@
+HTTP 529
+content-type: application/json
+
+{"type":"error","error":{"type":"overloaded_error","message":"Overloaded"},"request_id":"req_011CSHoEeqs5C35K2UUqR7Fz"}

packages/halkyon_llm/test/fixtures/documented/anthropic/text.http

@@ -0,0 +1,39 @@
+HTTP 200
+content-type: text/event-stream; charset=utf-8
+
+event: message_start
+data: {"type":"message_start","message":{"id":"msg_01Xq7aVbN3T9kd2LwPZr5HcE","type":"message","role":"assistant","model":"claude-sonnet-5-5","content":[],"stop_reason":null,"stop_sequence":null,"usage":{"input_tokens":412,"cache_creation_input_tokens":0,"cache_read_input_tokens":0,"output_tokens":1}}}
+
+event: content_block_start
+data: {"type":"content_block_start","index":0,"content_block":{"type":"thinking","thinking":"","signature":""}}
+
+event: ping
+data: {"type": "ping"}
+
+event: content_block_delta
+data: {"type":"content_block_delta","index":0,"delta":{"type":"thinking_delta","thinking":""}}
+
+event: content_block_delta
+data: {"type":"content_block_delta","index":0,"delta":{"type":"signature_delta","signature":"EqQBCgIYAhIMxk3Lr9vQm2HbW7pTGgy1d2nXo8Rf4KsJ0eQiMNp6"}}
+
+event: content_block_stop
+data: {"type":"content_block_stop","index":0}
+
+event: content_block_start
+data: {"type":"content_block_start","index":1,"content_block":{"type":"text","text":""}}
+
+event: content_block_delta
+data: {"type":"content_block_delta","index":1,"delta":{"type":"text_delta","text":"„Hund“ heißt auf Japanisch"}}
+
+event: content_block_delta
+data: {"type":"content_block_delta","index":1,"delta":{"type":"text_delta","text":" 犬 (いぬ, inu)."}}
+
+event: content_block_stop
+data: {"type":"content_block_stop","index":1}
+
+event: message_delta
+data: {"type":"message_delta","delta":{"stop_reason":"end_turn","stop_sequence":null},"usage":{"output_tokens":38}}
+
+event: message_stop
+data: {"type":"message_stop"}
+

packages/halkyon_llm/test/fixtures/documented/anthropic/tool.http

@@ -0,0 +1,45 @@
+HTTP 200
+content-type: text/event-stream; charset=utf-8
+
+event: message_start
+data: {"type":"message_start","message":{"id":"msg_014p7gG3wDgGV9EUtLvnow3U","type":"message","role":"assistant","model":"claude-sonnet-5-5","stop_sequence":null,"usage":{"input_tokens":472,"output_tokens":2},"content":[],"stop_reason":null}}
+
+event: content_block_start
+data: {"type":"content_block_start","index":0,"content_block":{"type":"text","text":""}}
+
+event: ping
+data: {"type": "ping"}
+
+event: content_block_delta
+data: {"type":"content_block_delta","index":0,"delta":{"type":"text_delta","text":"Ich sehe"}}
+
+event: content_block_delta
+data: {"type":"content_block_delta","index":0,"delta":{"type":"text_delta","text":" im Studienset nach."}}
+
+event: content_block_stop
+data: {"type":"content_block_stop","index":0}
+
+event: content_block_start
+data: {"type":"content_block_start","index":1,"content_block":{"type":"tool_use","id":"toolu_01T1x1fJ34qAmk2tNTrN7Up6","name":"find_cards","input":{}}}
+
+event: content_block_delta
+data: {"type":"content_block_delta","index":1,"delta":{"type":"input_json_delta","partial_json":""}}
+
+event: content_block_delta
+data: {"type":"content_block_delta","index":1,"delta":{"type":"input_json_delta","partial_json":"{\"query\":"}}
+
+event: content_block_delta
+data: {"type":"content_block_delta","index":1,"delta":{"type":"input_json_delta","partial_json":" \"Hu"}}
+
+event: content_block_delta
+data: {"type":"content_block_delta","index":1,"delta":{"type":"input_json_delta","partial_json":"nd\", \"limit\": 5}"}}
+
+event: content_block_stop
+data: {"type":"content_block_stop","index":1}
+
+event: message_delta
+data: {"type":"message_delta","delta":{"stop_reason":"tool_use","stop_sequence":null},"usage":{"output_tokens":89}}
+
+event: message_stop
+data: {"type":"message_stop"}
+