feat: add generic OIDC detector

[cloudsmith-iduffy]

What

Adds a generic fallback OIDC detector that reads an OIDC JWT from the CLOUDSMITH_OIDC_TOKEN environment variable.

This supports:

• Jenkins (via the credentials binding plugin)
• Any custom CI/CD system that can inject an OIDC token via an environment variable

How

• New GenericDetector in cloudsmith_cli/core/credentials/oidc/detectors/generic.py, following the same EnvironmentDetector pattern as the AWS detector.
• detect() returns True when CLOUDSMITH_OIDC_TOKEN is set and non-empty; get_token() returns the value or raises ValueError with guidance if unset.
• Registered last in _DETECTORS so vendor-specific detectors (AWS, etc.) take precedence — the generic detector is a catch-all fallback.
• The token flows through the existing exchange path unchanged.

Naming note

The environment variable name CLOUDSMITH_OIDC_TOKEN was considered against alternatives (CLOUDSMITH_OIDC_ID_TOKEN, CLOUDSMITH_OIDC_JWT) since "TOKEN" could be misread as a directly-usable API token rather than a JWT to be exchanged. The shorter CLOUDSMITH_OIDC_TOKEN was chosen; the get_token() error message clarifies it is "the OIDC JWT to exchange for a Cloudsmith token."

Testing

• Added cloudsmith_cli/core/tests/test_generic_detector.py (TDD — written first), mirroring the Bitbucket detector tests: detection (present / unset / empty) and token retrieval (returns / raises).
• All new tests pass; related credential/detector suite (43 tests) passes.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds a generic, env-var based OIDC detector so the CLI can exchange an injected OIDC JWT (e.g., from Jenkins or custom CI/CD) when no vendor-specific detector matches.

Changes:

• Introduces GenericDetector that reads CLOUDSMITH_OIDC_TOKEN for OIDC token exchange.
• Registers the generic detector after existing detectors so it acts as a fallback.
• Adds unit tests covering presence/absence of the env var and token retrieval behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
cloudsmith_cli/core/credentials/oidc/detectors/generic.py	Adds a new fallback detector that sources an OIDC JWT from CLOUDSMITH_OIDC_TOKEN.
cloudsmith_cli/core/credentials/oidc/detectors/__init__.py	Registers the new detector in the detector chain.
cloudsmith_cli/core/tests/test_generic_detector.py	Adds tests for generic detector detection and token retrieval behavior.

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[BartoszBlizniak]

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