[feat](distributed) Add scoped secret plan references

[kaka11chen]

Summary

• Add a versioned, opaque ScopedSecretRef contract with deterministic UUIDv8 identity, query/session ownership, DuckDB-selected normalized scope, and read/write capabilities; secret names and credential values never enter the plan or pickle state.
• Discover only referenced object-storage uses, retain both matched and initially-unmatched source uses for transport-time freshness checks, and propagate refs through logical/physical planning, strict pickle state, clones, deferred execution, query replay, and worker task reconstruction.
• Prove generated remote COPY credential selection across both the canonical data namespace and the sibling .duckdb_commit lifecycle namespace; hard-reject nonstandard lookup semantics, nested selection changes, stale registry state, unsupported providers, ambiguous refs, malformed input, and resource-limit violations.
• Add metadata-only DuckDB secret scanning that does not clone credential-bearing entries, standard lookup certificates, storage-registry generation checks, bounded/redacted validation, and custom DUCKDB_SOURCE_PATH-correct mbedTLS includes.

This is the inactive discovery/transport foundation from #588. It does not resolve, refresh, or install credentials on Ray workers and does not yet enable user-visible multi-scope distributed I/O; worker resolution remains a later broker task. No compatibility or ambient-credential fallback state is carried.

Related issue

close: #588

Documentation impact

• No user-facing documentation update is required.
• Documentation is updated in this PR or in a linked website PR.
• A follow-up issue is required in
  AstroVela/vane-website.

The contract remains internal and inactive until the worker broker/resolution work is implemented.

Validation

• Two consecutive independent clean review rounds on the unchanged final HEAD after all review findings were fixed; each round covered C++ API/call-site closure, ref propagation and serialization, COPY data/commit paths, selection proofs, concurrency, limits, and redaction.
• scripts/format workspace --changed
• Non-editable incremental Release install with SKBUILD_BUILD_DIR="$PWD/build/python-release", SKBUILD_CMAKE_BUILD_TYPE=Release, and uv pip install . --no-build-isolation.
• DuckDB unittest "[secret]" — 14 test cases passed, 159 assertions; 5 S3-server-dependent cases skipped.
• scripts/run_installed_pytest.sh tests/fast/test_scoped_secret_refs.py tests/fast/test_ray_connection_snapshot.py — 84 passed.
• scripts/run_release_tests.sh — non-Ray: 553 passed, 15 deselected; real Ray: 11 passed, 557 deselected.

Checklist

• The change is focused and includes tests or a reason tests are unnecessary.
• Public behavior and compatibility impact are documented.
• New dependencies, copied code, model assets, and datasets have compatible
  licenses and are recorded where required.
• No credentials, private endpoints, personal paths, generated data, model
  weights, or build artifacts are included.
• Security implications of UDFs, serialization, remote code, network access,
  and untrusted input have been considered.
• Native changes were compiled; Python-only, documentation, and workflow
  changes passed relevant checks.

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[chatgpt-codex-connector]

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[kaka11chen]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. 👍

Reviewed commit: 9d91c17ba6

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