feat(rc-admin)!: add bounded diagnostic snapshots

[cxymds]

Related issue

Closes rustfs/backlog#1400

Stacked on #282 (cxymds/issue-1403-diagnostic-capabilities).

Background and user impact

RustFS beta.10 exposes authenticated health information, a v4 cluster snapshot, and an extension catalog, but rc did not provide bounded typed commands for those routes. Route presence alone is insufficient: unknown server versions must remain fail-closed unless a read-only route is successfully confirmed, malformed/partial telemetry must not become a zero-valued success, and snapshot: null must remain distinct from a missing response field.

Solution

• Add rc admin diagnostics health|cluster|extensions ALIAS.
• Add forward-compatible core snapshot types that preserve unknown fields.
• Bound all diagnostic and capability-discovery success/error bodies to 8 MiB, including /v3/info and all three v4 discovery reads.
• Reconcile successfully probed cluster and extension routes to Available on unknown versions while keeping health conservative.
• Reject semantically incomplete health, cluster, and extension payloads.
• Represent explicit snapshot: null as initializing_or_unavailable, never healthy.
• Keep extension ordering deterministic and never access extension instances or speedtest routes.
• Add distinct output-v3 types: health_snapshot, cluster_snapshot, and extension_catalog.

BREAKING contract note

BREAKING: This PR updates the protected docs/reference/rc/ behavior contract and adds three output-v3 enum values for new commands. It does not change output v1/v2, config schema, exit-code definitions, or existing command behavior. Migration is opt-in: consumers only see the new schemas when invoking the new diagnostics commands.

Validation

Exact head: 1fb67e15de3f1bb300b110afe3d9864c752e5cce

• cargo fmt --all --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace
• Focused core diagnostic contract: 6 passed
• Focused S3 diagnostic reads: 3 passed
• Focused CLI diagnostics: 4 passed
• Independent review and post-fix re-review: no remaining findings

Limitations

• This is not mc support diag archive parity.
• Drive observations remain in rustfs/backlog#1407 and reuse the observability branch.
• Object/network/site speedtests, inspect archive retrieval, SUBNET upload, and extension instance configuration are out of scope.
• A local live beta.10 container run was not possible because the workspace Docker daemon is unavailable; validation is source-backed plus mock/integration test coverage until GitHub checks complete.

[cxymds]

Validation is complete for exact head 1fb67e15de3f1bb300b110afe3d9864c752e5cce.

Local gates:

• cargo fmt --all --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace

Focused validation:

• Core diagnostic contract: 6 passed.
• S3 bounded diagnostic reads: 3 passed.
• CLI diagnostics and semantic-invalid exit mapping: 4 passed.
• Independent review plus post-fix re-review: no remaining findings.

Remote gates:

• CI: https://github.com/rustfs/cli/actions/runs/29850707271 — green, including Linux/macOS/Windows build and tests, MSRV, docs, advisories, Clippy, and protected-file policy.
• Integration Tests: https://github.com/rustfs/cli/actions/runs/29850707252 — green.

The draft PR is open and its true stacked base is restored to cxymds/issue-1403-diagnostic-capabilities. The protected documentation/output-v3 additions are declared as BREAKING in the title and description.

A local live beta.10 container run was not performed because the Docker daemon is unavailable in this workspace; the validation claim is limited to the source-backed contract audit, mocks, local suites, and GitHub checks above.