feat: add Bitbucket Pipelines OIDC detector

[cloudsmith-iduffy]

What

Adds an OIDC environment detector for Bitbucket Pipelines, following the same pattern as the existing GitHub Actions / Azure DevOps / GitLab CI detectors.

Bitbucket injects an OIDC JWT into BITBUCKET_STEP_OIDC_TOKEN when a pipeline step sets oidc: true. The detector reads that variable and hands it to the standard Cloudsmith token exchange (/openid/{org}/ with service_slug); no audience handling is needed since Bitbucket sets the audience itself.

• bitbucket_pipelines.py — detect() checks the env var, get_token() returns it (clear error when absent)
• Registered in the detector chain ahead of AWS (cheap, specific env-var match before boto3 credential discovery)
• 5 unit tests (TDD), full core suite green, all pre-commit hooks pass

Live validation

Verified end-to-end against a real Bitbucket pipeline (org iduffy-demo, service bitbucket-y3j7) — build succeeded on a hosted runner, exercising detection → token fetch → Cloudsmith exchange → authenticated API call.

Token claims minted by Bitbucket:

iss = https://api.bitbucket.org/2.0/workspaces/cloudsmith-iduffy/pipelines-config/identity/oidc
aud = ari:cloud:bitbucket::workspace/<workspace-uuid>   (Bitbucket-set, not "cloudsmith")
sub = {repo-uuid}:{step-uuid}

cloudsmith whoami --verbose in the pipeline:

User: bitbucket (slug: bitbucket-y3j7)
Authentication Method: OIDC Auto-Discovery
Source: OIDC via Bitbucket Pipelines (org: iduffy-demo, service: bitbucket-y3j7)

cloudsmith list repos iduffy-demo then returned repositories — confirming the exchanged token authenticates real API calls.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

• Adds a Bitbucket Pipelines OIDC environment detector to the existing OIDC auto-discovery flow so the CLI can exchange Bitbucket-provided step OIDC JWTs for Cloudsmith API tokens.

Changes:

• Introduces BitbucketPipelinesDetector that reads BITBUCKET_STEP_OIDC_TOKEN from the environment.
• Registers the new detector in the OIDC detector chain ahead of AWS detection.
• Adds unit tests covering detect/get_token behavior for the Bitbucket detector.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
cloudsmith_cli/core/tests/test_bitbucket_pipelines_detector.py	Adds unit tests for Bitbucket Pipelines detection and token retrieval.
cloudsmith_cli/core/credentials/oidc/detectors/bitbucket_pipelines.py	Implements the Bitbucket Pipelines OIDC environment detector.
cloudsmith_cli/core/credentials/oidc/detectors/init.py	Registers Bitbucket Pipelines detector in the auto-discovery chain.

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