[BUG](tracing): Ignore blank OTEL endpoint

[anishfyi]

Description

Treat empty and whitespace-only OpenTelemetry endpoints as disabled instead of passing them to the OTLP exporter and panicking on an invalid URI.

The tracing layer now returns an identity layer for blank endpoints, so stdout tracing can continue without attempting OTLP exporter initialization.

Closes #4609

Test plan

• cargo test -p chroma-tracing
• cargo test -p chroma-tracing --doc
• cargo clippy -p chroma-tracing --tests -- -D warnings
• cargo fmt --all -- --check

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

[github-actions]

Reviewer Checklist

Please leverage this checklist to ensure your code review is thorough before approving

Testing, Bugs, Errors, Logs, Documentation

• Can you think of any use case in which the code does not behave as intended? Have they been tested?
• Can you think of any inputs or external events that could break the code? Is user input validated and safe? Have they been tested?
• If appropriate, are there adequate property based tests?
• If appropriate, are there adequate unit tests?
• Should any logging, debugging, tracing information be added or removed?
• Are error messages user-friendly?
• Have all documentation changes needed been made?
• Have all non-obvious changes been commented?

System Compatibility

• Are there any potential impacts on other parts of the system or backward compatibility?
• Does this change intersect with any items on our roadmap, and if so, is there a plan for fitting them together?

Quality

• Is this code of a unexpectedly high quality (Readability, Modularity, Intuitiveness)