Add config-based OAuth client IDs

[mduongvandinh]

Summary

• Users can configure real OAuth client IDs via [oauth] section in config.toml
• Falls back to built-in placeholder IDs when not configured

Changes

• Add OAuthConfig struct to KernelConfig with 4 optional provider fields
• Add client_ids() function accepting Option<&str> overrides per provider
• Refactor default_client_ids() to delegate to client_ids(None, None, None, None)
• Add 3 tests verifying override and fallback behavior

Config example

[oauth]
google_client_id = "123456789.apps.googleusercontent.com"
github_client_id = "Iv1.abc123def456"

Test plan

• All 1710+ workspace tests pass
• cargo clippy -p openfang-extensions -p openfang-kernel --all-targets -- -D warnings zero warnings
• cargo fmt --all --check clean
• 8 OAuth tests pass including 3 new ones

Files changed

• crates/openfang-types/src/config.rs (+25)
• crates/openfang-extensions/src/oauth.rs (+72, -13)