Fix LangChain tracer main-agent attribute propagation timing

[hectorhdzg]

The LangChain tracer's _start_trace() was setting gen_ai.agent.* attributes on the wrapper span AFTER creating the inner child span. This meant GenAIMainAgentSpanProcessor.on_start() fired for the child before the parent had any agent attributes, so microsoft.gen_ai.main_agent.* never propagated to LLM/tool spans.

Fix: move all agent attribute set_attribute() calls on the wrapper span to immediately after its creation, before the inner span is started.

Also adds:

• tests/genai/main_agent/test_sdk_propagation.py: real SDK span tests covering all GenAI span patterns (invoke_agent -> chat/tool, multi-agent, two-span wrapper, siblings, timing bug demonstration)
• tests/langchain/test_main_agent_propagation.py: LangChain-specific end-to-end integration tests and attribute key compatibility checks

[Copilot]

Pull request overview

This PR fixes a timing issue in the LangChain tracer where gen_ai.agent.* attributes were being set on the agent wrapper span after the inner (child) span was created, preventing GenAIMainAgentSpanProcessor.on_start() from propagating microsoft.gen_ai.main_agent.* onto downstream spans. The change ensures agent attributes are applied to the wrapper span immediately after wrapper creation and before starting the inner span, restoring correct main-agent propagation.

Changes:

• Move wrapper-span agent attribute setting earlier in LangChainTracer._start_trace() so child spans see the parent’s agent attributes at on_start.
• Add real OpenTelemetry SDK propagation tests covering multiple span-hierarchy patterns and the original timing bug.
• Add LangChain tracer integration tests validating end-to-end main-agent propagation and constant/key compatibility.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/microsoft/opentelemetry/_genai/_langchain/_tracer.py	Sets agent attributes on the wrapper span before creating the inner span to allow GenAIMainAgentSpanProcessor.on_start() propagation.
tests/genai/main_agent/test_sdk_propagation.py	Adds real-SDK tests validating main-agent propagation across common GenAI span patterns and reproducing the timing failure scenario.
tests/langchain/test_main_agent_propagation.py	Adds LangChain tracer end-to-end propagation tests plus attribute-key compatibility assertions against a365 constants.

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