Commonize OIDC handler test cases with table-driven generation

Replace 65 hand-written test cases (13 ecosystems × 5 providers) with
table-driven generation using buildEcosystemCases helper. Each ecosystem
defines its variants compactly — provider credential fields are shared
via oidcProviderFields, and expected log lines / auth URLs are derived
automatically with explicit overrides for special cases.

Reduces oidc_handling_test.go from 1846 to 685 lines (~63%) while
preserving identical test coverage: same 65 test cases, same test names,
same assertions, same provider mock endpoints.

Special cases handled explicitly:
- Docker/Helm JFrog: host-derived log targets
- Go proxy/Terraform Azure: host-based credentials
- Python GCP: index-url with /simple suffix stripping
- RubyGems: dual url+host credential fields
- NuGet: service index discovery mocks and resource URL registration
- Composer JFrog: dual registry+url fields

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: kbukum1