Switch to path filtering instead of conformance markers to run conformance tests

[lmolkova]

And fix CI broken in #10

[Copilot]

Pull request overview

This PR updates how GenAI semantic-convention conformance tests are selected/executed so non-conformance test environments don’t import conformance-only dependencies, addressing CI breakage referenced in #10.

Changes:

• Switch conformance selection from a pytest.mark.conformance marker to running tests/test_conformance.py directly in *-conformance tox envs, and --ignore-ing it in regular envs.
• Make OTLP/gRPC exporter imports lazy in the shared conformance runner so the module can be imported without conformance-only deps installed.
• Remove the now-unused conformance pytest marker registration and module-level marker usage; update AGENTS.md accordingly.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
util/opentelemetry-test-util-genai/src/opentelemetry/test_util_genai/conformance.py	Lazily imports OTLP/gRPC exporters and updates docs to reflect path-based conformance execution.
tox.ini	Runs conformance tests by targeting tests/test_conformance.py directly and ignores it in non-conformance envs.
pytest.ini	Removes the unused conformance marker registration.
instrumentation/opentelemetry-instrumentation-openai-v2/tests/test_conformance.py	Drops module-level conformance marker (no longer needed with path-based selection).
instrumentation/opentelemetry-instrumentation-langchain/tests/test_conformance.py	Drops module-level conformance marker (no longer needed with path-based selection).
instrumentation/opentelemetry-instrumentation-anthropic/tests/test_conformance.py	Drops module-level conformance marker (no longer needed with path-based selection).
AGENTS.md	Updates contributor guidance to match the new tox/path-based conformance execution model.