refactor(runner-providers): centralize dynamic label selection

[edersonbrilhante]

Description

• centralize dynamic-label enablement, provider targeting, queue fallback, logging, and dispatch-target construction in the shared webhook selector
• narrow DynamicLabelProvider to provider-owned validation through getViolations
• keep EC2 legacy-policy resolution and MicroVM parsing/policy validation inside their provider packages
• test shared queue selection through injected fake providers while retaining concrete validation tests inside each provider
• test cross-provider label ownership dynamically from the registered provider type list

This PR is stacked on #5255 and should be reviewed after it.

Test Plan

• yarn vitest run (58 files, 869 tests passed)
• yarn eslint libs/runner-providers
• yarn prettier --check "libs/runner-providers/**/*.ts"
• yarn workspace @aws-github-runner/webhook build
• git diff --check

Related Issues

Depends on #5255.

[github-actions]

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

Scanned Files

None