otel: add host.name resource attribute to logs/traces via gethostname (#12352)

**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`

codex-rs/Cargo.lock

@@ -2095,6 +2095,7 @@ dependencies = [
  "codex-utils-absolute-path",
  "codex-utils-string",
  "eventsource-stream",
+ "gethostname",
  "http 1.4.0",
  "opentelemetry",
  "opentelemetry-appender-tracing",