fix(telemetry): break helpers->core import cycle so metrics plugins register

[ajbozarth]

Issue

Fixes #1079

Description

mellea/helpers loaded mellea.core at module level, creating a cycle when mellea.core.utils -> mellea.telemetry -> metrics' bottom-of-module registration -> mellea.helpers ran during core init. The except ImportError swallowed the cycle as "plugin framework is not installed", so MELLEA_METRICS_ENABLED=true registered zero plugins in production.

• helpers/async_helpers.py and openai_compatible_helpers.py: move core imports under TYPE_CHECKING or into function bodies.
• telemetry/metrics.py: replace except ImportError with explicit _HAS_PLUGIN_FRAMEWORK check; move registration block below __all__.

The lazy-init migration originally scoped in #1079 is declined in favor of eager-parity with the upcoming tracing plugin design.

Testing

• Tests added to the respective file if code was changed
• New code has 100% coverage if code was added
• Ensure existing tests and github automation passes (a maintainer will kick off the github automation when the rest of the PR is populated)

Attribution

• AI coding assistants used

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[ajbozarth]

So while wrapping up work on phase 1 of #444 I discovered a chicken and egg issue with registering plugins that makes fully lazy init not work. In addition I found that this bug also affected the tracing work. So I decided to quickly open this bug fix since it was more severe than I thought (essentially metrics has been non-functional since I switched it to use hooks). The remaining items in #1079 are now won't fix

[planetf1]

LGTM — reproduced on the parent (0 plugins), confirmed all 8 register here.

Two non-blocking follow-ups:

• metrics_plugins.py:18 imports from mellea.core.base import GenerationMetadata at module level — the same cycle this PR fixes, safe only because core/__init__.py imports .base before .utils. If that order ever changes the cycle reopens, now silently (since this PR removes the loud failure). Worth a follow-up to make plugin registration lazy and drop the dependency on import order.
• A regression test in test/package/test_dependency_isolation.py (already runs isolated interpreters) asserting the plugin count is non-zero with metrics enabled would help — the existing plugin tests use mocks, so they couldn't catch this failure mode.