docs: replace retired claude-sonnet-4-5 in examples and docstring (#1070)

The langchain prompt-cache example and two `claude_agent_sdk` docstring
snippets default to `claude-sonnet-4-5-20250929`, which Anthropic
retires on 2026-11-30. After that date, copying the example fails with a
model-not-found error.

Changes: use `claude-sonnet-5`, the Sonnet alias already used in
`test_anthropic.py`, in `examples/langchain/anthropic_prompt_cache.py`
and the two docstring snippets in `claude_agent_sdk/__init__.py`.
Recorded VCR cassettes, and the test that replays them
(`langchain/test_callbacks.py`), keep the old ID because they are
historical recordings.

Test: no behaviour change; both files compile, and the old ID now
appears only in cassette-backed tests.

This change was prepared with the assistance of an AI coding tool
(Claude) and reviewed before submission.

Co-authored-by: Claude Sonnet 5.5 <noreply@anthropic.com>

Author: pcbeingused333

examples/langchain/anthropic_prompt_cache.py

@@ -22,7 +22,7 @@
 load_dotenv(ROOT / ".env")
 
 PROJECT_NAME = os.environ.get("BRAINTRUST_PROJECT", "py-sdk-demo-langchain-anthropic-cache")
-MODEL = os.environ.get("ANTHROPIC_MODEL", "claude-sonnet-4-5-20250929")
+MODEL = os.environ.get("ANTHROPIC_MODEL", "claude-sonnet-5")
 
 # Anthropic prompt caching requires a sufficiently long cacheable prefix.
 CACHEABLE_SYSTEM_PROMPT = "\n".join(

py/src/braintrust/integrations/claude_agent_sdk/__init__.py

@@ -7,7 +7,7 @@
     setup_claude_agent_sdk(project="my-project")
 
     # Use normally - all calls are automatically traced
-    options = ClaudeAgentOptions(model="claude-sonnet-4-5-20250929")
+    options = ClaudeAgentOptions(model="claude-sonnet-5")
     async with ClaudeSDKClient(options=options) as client:
         await client.query("Hello!")
         async for message in client.receive_response():
@@ -50,7 +50,7 @@ def setup_claude_agent_sdk(
         setup_claude_agent_sdk(project="my-project")
 
         # Now use claude_agent_sdk normally - all calls automatically traced
-        options = claude_agent_sdk.ClaudeAgentOptions(model="claude-sonnet-4-5-20250929")
+        options = claude_agent_sdk.ClaudeAgentOptions(model="claude-sonnet-5")
         async with claude_agent_sdk.ClaudeSDKClient(options=options) as client:
             await client.query("Hello!")
             async for message in client.receive_response():