refactor: reduce complexity of execute_impl in upload_pipeline_artifact.rs (#1625)

Extract six focused helpers from the 383-line execute_impl:
- resolve_effective_build_id: resolve build ID from request or BUILD_BUILDID
- resolve_final_artifact_name: all allowlist/name-prefix/extension checks
- resolve_staged_file: canonicalize, bounds-check, size/dir checks
- resolve_ado_context: extract org_url/project/project_id/token/container_id
- upload_file_to_container: Step 1 HTTP PUT to the build container
- associate_artifact_with_build: Step 2 HTTP POST to associate artifact record

The remaining execute_impl is now 85 lines with a clear sequential narrative:
resolve IDs → validate names → validate staged file → dry-run → verify hash
→ resolve context → dedupe check → upload → associate → record dedupe key.

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Author: github-actions[bot]