Simplify repeated Docker replacement test environment setup

The Docker replacement cleanup suite repeats the same environment key inventory, save/clear logic and restoration in five transport-backed flows. This duplicates security-sensitive fixture isolation and obscures the actual replacement failures under test. The error-classifier suite also repeats the identical SSH connect-timeout input and result.

Simplify the repeated environment setup locally within this suite, retaining each case's exact controlled keys and values and restoration on failure. Remove only the exact duplicate classifier case. Keep funding, callback ordering, identity, crash recovery, registration and cleanup failure coverage. Do not introduce a general fixture framework or change production behavior.

Acceptance: fewer maintained test lines, unchanged unique behaviors, retained replacement and classifier suites passing together, package type/lint and root verification passing. Existing complete provider and negative-path cases own these contracts; no replacement tests are needed for fixture deduplication.