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.

[hemasekhar-p]

Hi @r4inee, Thank you for taking time and submitting this PR. Currently this PR is under review by our team, we will keep you posted if any additional information is required. thank you.

[hemasekhar-p]

@tilgalas, Could you please review this.