fix(observability): skip OTLP-JSON exporter tests without the otel extra

[abhati27]

Description

uv run pytest src/ -k "not integration" fails on a fresh clone with two errors in
src/observability/tests/test_file_exporter.py, both
ModuleNotFoundError: No module named 'google.protobuf'.

Fix Details

The two encoding tests go through _encode_otlp_json, which imports protobuf and the
OTLP common trace encoder. Both come from the optional otel dependency group, so a
default uv sync doesn't install them. Gated those two tests with
pytest.importorskip so they skip when the extra is absent and still run when it's
present. test_file_exporter_creates_parent_dir is left alone since it doesn't encode
anything.

This follows what INSTRUCTIONS.md already describes for the server suites, where a
missing dependency skips rather than fails.

Impact on Benchmarking

• No change to baselines: This fix only improves stability/performance.

Test-only change, no scorer or runtime code touched.

Related Issues

• Fixes: no existing issue, found while setting the repo up locally.

Verification Steps

1. uv run pytest src/ -k "not integration"

   install	before	after
   default uv sync	8 failed, 696 passed, 3 skipped	6 failed, 696 passed, 5 skipped

   The remaining 6 are unrelated, in test_static_json_scorer.py, which passes an
   evaluation_metadata= kwarg that evaluate_static_json doesn't accept.

2. Checked both directions so the guard isn't a permanent mute:

   • uv sync --group otel, then pytest src/observability/tests/test_file_exporter.py → 3 passed
   • uv sync, then the same → 1 passed, 2 skipped
   • uvx ruff check and uvx ruff format --check clean on the changed file

Checklist

• I have added tests that prove my fix is effective — this change is to the tests
  themselves, verified by running them with and without the otel extra, as above.
• My code follows the project's Ruff formatting and linting rules.
• I have signed off my commits (DCO).

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[DhavalRepo18]

@abhati27 Whould you like to create a git issue first.