feat(rc-admin)!: add scanner, storage, and metrics diagnostics

[cxymds]

Related issue

Closes rustfs/backlog#1376

Background and impact

RustFS beta.10 exposes scanner status, storage information, and realtime metrics through Admin API v3, but rc had no typed client boundary or safe CLI surface for these diagnostics. Operators could not distinguish stale, empty, partial, unauthorized, and unsupported states in automation.

Solution

• add typed core contracts for scanner, storage, and realtime metric snapshots
• add signed Admin API clients for /scanner/status, /storageinfo, and streaming /metrics
• bound metrics to 120 snapshots, 1 MiB per record, and 16 MiB per response
• add rc admin scanner status, rc admin info storage, and rc admin metrics
• normalize metrics into v3 JSON Lines while preserving raw server snapshots, numeric values, labels, and timestamps
• distinguish permission failures from missing server routes
• extend output v3 schema, golden fixtures, help contracts, and reference documentation

BREAKING

This PR updates the protected rc admin behavior reference and extends the output v3 metrics contract with errors, partial/final markers, raw data, and per-sample timestamps. Consumers of the previously provisional metrics v3 shape must adopt the documented fields. Output v1 and v2 remain unchanged.

Validation

• cargo fmt --all --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace
• cargo test -p rustfs-cli --features integration --test integration --no-run

All validation commands pass locally.

[cxymds]

Exact-head validation is complete for 8ec668b483c600786df8d25871cd3387e2e36c3b.

• CI: all jobs passed, including formatting, Clippy, documentation, MSRV, dependency advisories, protected-file checks, tests on Ubuntu/macOS/Windows, and builds on Ubuntu/macOS/Windows: https://github.com/rustfs/cli/actions/runs/29825978119
• RustFS integration workflow passed: https://github.com/rustfs/cli/actions/runs/29825978030
• Local validation passed: cargo fmt --all --check, cargo clippy --workspace --all-targets -- -D warnings, and cargo test --workspace.

The PR remains draft because it is part of the ordered capability/schema stack.