Suppress redundant pyproject dependency warning for uv-managed projects

[codeforester]

Summary

• Suppress BASE-P142 when a project explicitly declares python.manager: uv.
• Preserve the warning for dependency metadata without an explicit supported Python manager.
• Add direct and JSON diagnostic regression coverage plus documentation and changelog updates.

Root Cause

The read-only pyproject.toml diagnostic classified all dependency metadata as unmanaged. That check ran independently of the manifest's explicit uv manager, even though Base delegates uv dependency synchronization and already reports uv readiness and drift through BASE-P150 through BASE-P155.

User Impact

Healthy uv-managed projects no longer look incomplete because of a redundant warning. Missing uv project files, environments, or synchronization drift remain visible through the dedicated uv findings.

Validation

• Focused pyproject and diagnostics tests: 28 passed, 1 skipped, 1 subtest passed.
• Full Python suite with TZ=UTC: 962 passed, 1 skipped, 254 subtests passed.
• Full source-checkout BATS suite: 870 passed.
• git diff --check passed.
• Pylint was unavailable because it is not installed in the environment.

Demo Impact

None. This is a diagnostics-only behavior fix.

AI Context

No .ai-context/ update is needed; the change refines an existing diagnostic boundary without changing Base's product shape, command surface, or manifest model.

Fixes #1968