otel: add host.name resource attribute to logs/traces via gethostname

[mcgrew-oai]

PR Summary

This PR adds the OpenTelemetry host.name resource attribute to Codex OTEL exports so every OTEL log (and trace, via the shared resource) carries the machine hostname.

What changed

• Added host.name to the shared OTEL Resource in /Users/michael.mcgrew/code/codex/codex-rs/otel/src/otel_provider.rs
  • This applies to both:
    • OTEL logs (SdkLoggerProvider)
    • OTEL traces (SdkTracerProvider)
• Hostname is now resolved via gethostname::gethostname() (best-effort)
  • Value is trimmed
  • Empty values are omitted (non-fatal)
• Added focused unit tests for:
  • including host.name when present
  • omitting host.name when missing/empty

Why

• host.name is host/process metadata and belongs on the OTEL resource, not per-event attributes.
• Attaching it in the shared resource is the smallest change that guarantees coverage across all exported OTEL logs/traces.

Scope / Non-goals

• No public API changes
• No changes to metrics behavior (this PR only updates log/trace resource metadata)

Dependency updates

• Added gethostname as a workspace dependency and codex-otel dependency
• Cargo.lock updated accordingly
• MODULE.bazel.lock unchanged after refresh/check

Validation

• just fmt
• cargo test -p codex-otel
• just bazel-lock-update
• just bazel-lock-check