fix: Fix RxJava tracing context propagation

[r4inee]

Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: Telemetry: Plugins observe incorrect current span #1252
• Related: ADK Java OTel spans do not expose tool-call execution correctly in Langfuse/OpenInference #1235

Problem:
ADK Java plugin callbacks are not consistently invoked with the semantically correct OpenTelemetry span as Span.current(). This affects plugin authors who enrich ADK-created spans using:

Span.current().setAttribute(...);

Solution:

• Tracing.TracerProvider — Start spans before subscribing to upstream streams, instead of in doOnSubscribe. This ensures deferred RxJava sources see the tracing span as Span.current().

• Runner#runAsyncImpl — Capture Context.current() inside Flowable.defer(...) so the runner captures execution-time context, not stale assembly-time context.

• BaseAgent#run — Capture the parent context inside deferred execution so invoke_agent is correctly parented under the active invocation span.

• BaseLlmFlow#callLlm — Create the call_llm span before beforeModelCallback, so beforeModelCallback, model execution, afterModelCallback, and model error callbacks share the same call_llm span.

• BaseLlmFlow#callLlm — Re-scope emissions back to the parent context after the LLM segment, preventing follow-up tool/agent/LLM work from inheriting the previous call_llm context.

Testing Plan

• ContextPropagationTest — Add coverage proving Tracing.trace(...) starts spans before deferred upstream code runs.
• Existing coverage - ContextPropagationTest#testAgentWithToolCallTraceHierarchy and ContextPropagationTest#runnerRunAsync_propagatesContext which checks context propgation and hierarchy. Added testModelCallbacksObserveCallLlmSpan to check that the callbacks sees the correct current span.

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Manual End-to-End (E2E) Tests:

Hierarchy remains correct

With context propgation

Checklist

• I have read the CONTRIBUTING.md document.
• My pull request contains a single commit.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

[r4inee]

cc @krwc @mazas-google

[r4inee]

Tracing.TracerProvider — Start spans before subscribing to upstream streams, instead of in doOnSubscribe.

This ensures deferred RxJava sources see the tracing span as Span.current().

[r4inee]

Runner#runAsyncImpl — Capture Context.current() inside Flowable.defer(...) so the runner captures execution-time context, not stale assembly-time context.

[r4inee]

BaseAgent#run — Capture the parent context inside deferred execution so invoke_agent is correctly parented under the active invocation span.

[r4inee]

BaseLlmFlow#callLlm — Create the call_llm span before beforeModelCallback, so beforeModelCallback, model execution, afterModelCallback, and model error callbacks share the same call_llm span.

[r4inee]

BaseLlmFlow#callLlm — Re-scope emissions back to the parent context after the LLM segment, preventing follow-up tool/agent/LLM work from inheriting the previous call_llm context.