refactor(gax): organize observability utilities into util, contextUtil, and errorUtil

[shivanee-p]

Reorganizes the OpenTelemetry tracing helpers in google-gax (core/packages/gax/src/observability/) into focused utility modules and aligns the unit test suite accordingly.

Fixes:

• Extracted general observability helpers into observability/util.ts
• Consolidated trace context interfaces and active client-request/attempt context propagation into observability/contextUtil.ts
• Extracted error classification constants and error/exception helpers into observability/errorUtil.ts
• Split and deduplicated unit tests without redundant overlapping assertions

[gemini-code-assist]

Code Review

This pull request refactors the observability module in google-gax by extracting error handling, telemetry checking, and trace context utilities from TracerHelper.ts and util.ts into dedicated files: errorUtil.ts, contextUtil.ts, and observability/util.ts. Corresponding test files have been updated and expanded. Feedback on the changes suggests ensuring that detail.errorInfo.reason is validated as a non-empty string before returning it in resolveErrorInfoReason to prevent empty error.type attributes in telemetry.