Document Python agent versioning in SDK reference

[ncybul]

What does this PR do? What is the motivation?

Documents how to set an agent version when manually instrumenting agents with the Python SDK, so you can identify which version of an agent produced a span in the Agent Observability trace view. This documents the SDK support added in DataDog/dd-trace-py#19490.

All edits are in hugo/content/en/llm_observability/instrumentation/sdk.md, within the existing Python tabs:

• Agents: adds the optional version argument to the agent decorator's arguments, updates the example to @agent(version="v3"), and adds a context-manager example using LLMObs.agent(name="weather-agent", version="v3").
• Enriching spans: adds the optional agent argument (shape {"version": "..."}, also accepted through the ddtrace.llmobs.Agent typed dictionary) to the LLMObs.annotate() arguments, applied when annotating an agent span.
• Annotating auto-instrumented spans: adds the same optional agent argument to the LLMObs.annotation_context() arguments and adds an example that versions auto-instrumented agent spans.

How was this tested?

• Reviewed the rendered additions in context to confirm each one matches the surrounding argument lists and example blocks.
• Checked the new copy against the Datadog Vale substitution rules by hand (the Vale binary is not available in this environment).

Merge readiness

• Ready for merge

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AI assistance

Written with Bits Code (Claude Code): it made the documentation edits and drafted this PR description. The content was reviewed for accuracy.

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Modified Files

• https://docs-staging.datadoghq.com/dd/nicole.cybul/python-agent-versioning-docs/llm_observability/instrumentation/sdk