feat: resource_credits test suite, cargo-deny CI, expanded booking & token tests

[JoeX17]

Closes #1520, Closes #1521, Closes #1522, Closes #1523

• SC-03: Full test suite for resource_credits (20 tests from zero)
• SC-05: Added cargo-deny + cargo-audit to CI security scanning
• SC-06: Expanded workspace_booking tests from 15 to 48 tests
• SC-07: Expanded membership_token tests to 20 with bin target docs

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[yusuftomilola]

Solid test coverage expansion! Resource credits test suite, cargo-deny CI integration, and expanded booking/token tests improve code quality and dependency security.