feat(bitbucket): Add API-driven pipeline backend for Bitbucket integration setup

[evanpurkhiser]

Implement get_pipeline_api_steps() on BitbucketIntegrationProvider with
a single authorize step that verifies the Atlassian Connect JWT from
Bitbucket's addon authorization flow. The frontend posts the JWT received
from the popup callback, and the backend validates it against the original
callback path using get_integration_from_jwt. Updates build_integration()
to handle both legacy and API state paths.

Ref VDY-41

[linear-code]

VDY-41 Bitbucket: API-driven integration setup

[cursor]

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[github-actions]

Backend Test Failures

Failures on 9fe8c22 in this run:

tests/sentry/integrations/bitbucket/test_integration.py::BitbucketApiPipelineTest::test_invalid_jwt — log

[gw0] linux -- Python 3.13.1 /home/runner/work/sentry/sentry/.venv/bin/python3
tests/sentry/integrations/bitbucket/test_integration.py:342: in test_invalid_jwt
    assert "Unable to verify installation" in resp.data["jwt"][0]
E   KeyError: 'jwt'