Improve PerRequestSpanProcessor to allow set export token at later stage

[fpfp100]

Summary

Improve PerRequestSpanProcessor to allow set export token at later stage. Introduces updateExportToken() to allow refreshing the per-request export token within the active OTel async context. This is needed for long-running requests where the original token may expire before the trace is flushed.

Changes

• token-context.ts: Introduce TokenHolder interface so the token is stored as a mutable object reference in OTel Context (which is itself immutable). Add updateExportToken() to refresh the token in-place. Update getExportToken() with backward compatibility for raw string values from older callers.
• index.ts: Export updateExportToken from the package public API.
• PerRequestSpanProcessor.test.ts: Add e2e test verifying the refreshed token is used at export time under AsyncLocalStorageContextManager.

[Copilot]

Pull request overview

This PR introduces mutable token refresh capability for per-request observability exports by wrapping the token in a mutable TokenHolder object stored within the immutable OpenTelemetry Context. This enables long-running requests to refresh expired tokens before trace export without creating new contexts.

Changes:

• Introduced TokenHolder interface and updateExportToken() function to enable in-place token refresh within active OTel context
• Modified runWithExportToken() to store tokens in a TokenHolder wrapper instead of raw strings
• Enhanced getExportToken() with backward compatibility for legacy raw string values
• Exported updateExportToken() from the package public API
• Added integration test verifying refreshed tokens are used at export time

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
packages/agents-a365-observability/src/tracing/context/token-context.ts	Implements TokenHolder pattern for mutable token storage; adds updateExportToken() function; maintains backward compatibility in getExportToken()
packages/agents-a365-observability/src/index.ts	Exports new updateExportToken function from package public API
tests/observability/core/PerRequestSpanProcessor.test.ts	Adds integration test verifying token refresh works end-to-end with AsyncLocalStorageContextManager