refactor(ai): convert chat_sdk.ai module to package for chat/ai subpath (#116)

* refactor(ai): convert chat_sdk.ai module to package for chat/ai subpath (vercel/chat#492)

Mirror upstream PR vercel/chat#492 which split ai.ts into ai/messages.ts to
make room for ai/tools.ts. Convert the chat_sdk.ai module into a package:

- src/chat_sdk/ai/messages.py: to_ai_messages + supporting types, moved
  verbatim from the former ai.py (docstring path updated only).
- src/chat_sdk/ai/__init__.py: re-export shim exposing every symbol the old
  module exported, so from chat_sdk.ai import to_ai_messages keeps working.
- tests/test_ai.py renamed to tests/test_ai_messages.py (logic unchanged).

No public import path changed. PR 1 of 3 (module move only); the tool
factory (create_chat_tools) lands in PR 2.

Design issue #109, tracking #98.

https://claude.ai/code/session_01FyMxQn2BEAzmwKS1GZczKj

* fix(scripts): update AI test-fidelity mapping after module move (review)

* fix(ai): unbreak strict fidelity check + re-export TEXT_MIME_PREFIXES

Two CI / review follow-ups on PR #116:

1. Fidelity-mapping fix went too far in the previous commit. The script's
   parity pin is `chat@4.26.0` (see `lint.yml:63`), where the upstream
   AI test file is still the flat `packages/chat/src/ai.test.ts` — the
   move to `ai/messages.test.ts` happened in 4.29.0. Pointing the mapping
   at the 4.29-era path made `verify_test_fidelity.py --strict` fail with
   "upstream checkout missing — mapped TS file not found". Revert just
   the TS half of the mapping to `ai.test.ts`; keep the Python half at
   `tests/test_ai_messages.py` (the file this PR actually renamed). When
   the parity pin moves to 4.29.0 the TS path can advance too.

   Locally verified against a fresh `chat@4.26.0` clone:
       TOTAL: 564/564 matched (100%), 0 missing
       All TS tests have Python equivalents.

2. The package shim was missing `TEXT_MIME_PREFIXES` (flagged by Codex
   P2). The PR description promises to "re-export everything the former
   `chat_sdk.ai` module exposed" — add the constant to both the import
   block and `__all__` so the contract holds.

https://claude.ai/code/session_01FyMxQn2BEAzmwKS1GZczKj

---------

Co-authored-by: Claude <noreply@anthropic.com>

Author: patrick-chinchill

scripts/verify_test_fidelity.py

@@ -45,7 +45,7 @@
     "packages/chat/src/markdown.test.ts": "tests/test_markdown_faithful.py",
     "packages/chat/src/streaming-markdown.test.ts": "tests/test_streaming_markdown.py",
     "packages/chat/src/serialization.test.ts": "tests/test_serialization.py",
-    "packages/chat/src/ai.test.ts": "tests/test_ai.py",
+    "packages/chat/src/ai.test.ts": "tests/test_ai_messages.py",
     "packages/chat/src/from-full-stream.test.ts": "tests/test_from_full_stream.py",
 }
 

src/chat_sdk/ai/__init__.py

@@ -0,0 +1,37 @@
+"""AI SDK integration for the chat SDK.
+
+Python port of the ``chat/ai`` subpath. Mirrors the upstream structure where
+``ai.ts`` was split into ``ai/messages.ts`` (and, in later PRs, ``ai/tools.ts``)
+to make room for tool factories.
+
+Re-exports everything the former ``chat_sdk.ai`` module exposed so existing
+imports such as ``from chat_sdk.ai import to_ai_messages`` keep working.
+"""
+
+from __future__ import annotations
+
+from chat_sdk.ai.messages import (
+    TEXT_MIME_PREFIXES,
+    AiAssistantMessage,
+    AiFilePart,
+    AiImagePart,
+    AiMessage,
+    AiMessagePart,
+    AiTextPart,
+    AiUserMessage,
+    ToAiMessagesOptions,
+    to_ai_messages,
+)
+
+__all__ = [
+    "TEXT_MIME_PREFIXES",
+    "AiAssistantMessage",
+    "AiFilePart",
+    "AiImagePart",
+    "AiMessage",
+    "AiMessagePart",
+    "AiTextPart",
+    "AiUserMessage",
+    "ToAiMessagesOptions",
+    "to_ai_messages",
+]

src/chat_sdk/ai.py src/chat_sdk/ai/messages.pysrc/chat_sdk/ai.py renamed to src/chat_sdk/ai/messages.py

@@ -1,6 +1,6 @@
 """Convert chat Messages to AI SDK format.
 
-Python port of ai.ts.
+Python port of ``ai/messages.ts``.
 """
 
 from __future__ import annotations