feat(observability): add OTEL traces and logs to harnesses (#2805)

Adds correlated OpenTelemetry tracing and logging across the Kagent,
Codex, and Claude harnesses. Refactors shared tracing configuration and
setup code. Builds on top of previous V2 observability works like #2663
and #2786. Also closes #2760 and partially addresses #2762.

### Key Changes

- Resolve and validate OTLP trace and log settings consistently across
all harness compilers.
- Propagate telemetry configuration, service identity, and collector
egress into runtime revisions.
- Configure Codex and Claude native trace and log exporters with content
capture enabled.
- Propagate W3C trace context through Codex JSON-RPC requests and Claude
process environment.
- Initialize shared Go tracing in the native harness wrappers and move
common tracing code out of the ADK package.
- Flush request spans before terminal and input-required events cause an
Actor to pause.

### Notes

- Traces and logs are independently enabled through
`OTEL_TRACING_ENABLED` and `OTEL_LOGGING_ENABLED`
- Invalid or missing enabled-signal endpoints and unsupported protocols
now fail harness compilation.
- Span attributes for Claude Code is [documented
here](https://code.claude.com/docs/en/monitoring-usage#span-attributes)
and available events / logs is [documented
here](https://code.claude.com/docs/en/monitoring-usage#events)
- Events / logs emitted by Codex is [documented
here](https://learn.chatgpt.com/docs/config-file/config-advanced#what-gets-emitted).
There is no official docs for Codex's span hierarchy and attributes
- Codex and Claude Code force flushes traces when their process exists
before the gateway Suspends the instance. The adapter process will also
flush its own traces before it responds with a terminal event as per the
changes in `go/adk/pkg/a2a/server/server.go`. However, during a pending
HITL, since we do not quit the Codex or Claude process, it is not
guaranteed that all traces are delivered immediately.
- For both Codex and Claude, native spans after HITL completes remain in
the original request trace because the in-flight process carries the
same trace context (through env var or JSON-RPC). The resume A2A trace
may contain only controller and Go wrapper spans.

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Signed-off-by: Jet Chiang <pokyuen.jetchiang-ext@solo.io>