feat(admin): classify RustFS diagnostic capabilities

[cxymds]

Related issue

Closes rustfs/backlog#1403
Parent: rustfs/backlog#1383
Roadmap: rustfs/backlog#1361

Replaces closed #282 after its obsolete stacked base was removed.

Background and user impact

RustFS 1.0.0-beta.10 registers several diagnostic routes that are placeholders rather than real measurements. Route presence and HTTP 200 responses are therefore not sufficient evidence of support. Without probe-specific truth, rc could advertise or execute object, network, site, or site-replication performance tests that return misleading success.

The existing /v3/inspect-data endpoint returns reconstructed object bytes and is not an inspect archive.

Root cause

Runtime discovery exposed broad server capabilities but did not normalize individual diagnostic probe states or provide a reusable fail-closed guard. Unknown future server versions could not be distinguished safely from the pinned beta.10 contract.

Solution

• add 10 stable diagnostic capability names
• classify the pinned beta.10 contract:
  • available: health snapshot, cluster snapshot, extensions catalog, drive observations, and client-devnull
  • unsupported: inspect archive
  • stubbed: object, network, site, and site-replication netperf
• mirror cluster and extension availability from their actual read-only discovery requests
• treat unknown future versions as unknown and missing v4 discovery as version-gated
• preserve permission-denied, disabled, unsupported, stubbed, version-gated, and unknown states
• add a reusable CapabilityReport guard that permits only explicitly available diagnostics
• verify discovery never executes an active diagnostic or speed-test request

Validation

Passed locally at exact head 18c2fb5 synchronized with current main:

• cargo fmt --all --check
• CARGO_BUILD_JOBS=2 cargo clippy --workspace -- -D warnings
• CARGO_BUILD_JOBS=2 cargo test --workspace

Historical and current thread-aware audits found no reviews or unresolved review threads.

Review notes

This PR adds no diagnostic command and performs no active probe. Unknown versions remain fail-closed until a server-owned per-probe contract is available through rustfs/backlog#1402. Relative to current main, the implementation changes only core/S3 capability code and does not modify protected contracts.