Support declarative configuration via OTEL_CONFIG_FILE

[mmanciop]

What

Makes the distribution work with OpenTelemetry declarative (file-based) configuration:

• declarative-config extra: pip install dash0-opentelemetry-distro[declarative-config] installs the loader dependencies (opentelemetry-sdk[file-configuration], i.e. pyyaml + jsonschema). It stays an extra rather than a hard dependency because jsonschema drags in rpds-py (native wheels), which would break the distro's "no native dependencies, safe to inject" design goal.

• Preflight in Dash0Configurator: when OTEL_CONFIG_FILE is set but the loader can't be imported, log one actionable error and skip SDK configuration:

  OTEL_CONFIG_FILE is set but the declarative-configuration dependencies are not installed:
  File configuration requires pyyaml. Install with: pip install opentelemetry-sdk[file-configuration].
  The SDK will not be configured and no telemetry will be sent.
  

  Previously this failed with a raw ModuleNotFoundError traceback at startup, and the process silently ran without any telemetry.

The preflight checks only where the pinned SDK (1.43.0, from #8) keeps the loader; its docstring and the extra's comment flag that the 1.44 bump moves the loader to the separate opentelemetry-configuration package, so the future version-bump change knows to revisit both.

Validation

Verified end-to-end with opentelemetry-instrument against a local OTLP sink:

• With the extra: the SDK is built from the declarative YAML — spans (including the distro's DASH0_BOOTSTRAP_SPAN) export to the endpoint from the file, with the file's resource attributes; the vendored pyproto OTLP/HTTP exporter is picked up by the declarative otlp_http factory.
• Without the extra: the single error message above replaces the traceback and the application keeps running.
• New unit tests cover the skip-with-error path, normal delegation, that the import probe doesn't run without OTEL_CONFIG_FILE, and the preflight's message shape. Full suite: 32 passed via the CI command; ruff clean; uv.lock regenerated.

Known remaining gap (out of scope, tracked in #6): the distro's detected resource attributes are dropped under declarative configuration.

Fixes #5

🤖 Generated with Claude Code

[mmanciop]

Putting this on the backburner because we do not use declarative config in the Dash0 Operator right now, and it would bring the pyyaml and jsonschema dependencies, with the associated injection risks.