fix: conversation id override for baggage span processor

[LucasAlvesSoares]

Description

gen_ai.conversation.id was inconsistent across spans when using auto_instrument() with a LangGraph agent. The root cause: the opentelemetry-instrumentation-langchain callback handler calls
span.set_attribute("gen_ai.conversation.id", thread_id) mid-span, after on_start processors have already run, overwriting the caller-supplied value from the W3C baggage header.

The fix replaces the third-party opentelemetry-processor-baggage dependency with a built-in BaggageSpanProcessor that adds an on_end override mechanism. For keys in an allowlist
(gen_ai.conversation.id by default), the baggage value is snapshotted at on_start (the only point the OTel context is live) and re-applied to span._attributes at on_end, after all framework
callbacks have had their turn. All other baggage keys behave identically to before.

The two processor files are also moved into a span_processors/ subfolder for better organisation. This is an internal restructuring with no impact on the public API.

Related Issue

Closes #157

Type of Change

Please check the relevant option:

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Code refactoring
• Dependency update

How to Test

Describe how reviewers can test your changes:

1. Run a LangGraph agent with auto_instrument() and inject gen_ai.conversation.id in the W3C baggage header on the incoming request
2. Inspect the exported trace spans
3. Expected result: all spans — including the invoke_agent LangGraph span — carry the same gen_ai.conversation.id value that was injected by the calling system, not the LangGraph thread ID

Checklist

Before submitting your PR, please review and check the following:

• I have read the Contributing Guidelines
• I have verified that my changes solve the issue
• I have added/updated automated tests to cover my changes
• All tests pass locally
• I have verified that my code follows the Code Guidelines
• I have updated documentation (if applicable)
• I have added type hints for all public APIs
• My code does not contain sensitive information (credentials, tokens, etc.)
• I have followed Conventional Commits for commit messages

[jeanscherf]

Added a suggestion but not a blocker