feat(tencent): add --format json to clanker tencent list <type>

[rafeegnash]

Summary

Adds a --format flag (table | json, default table) to clanker tencent list <type>. JSON mode dispatches to the existing JSON* methods on *tencent.Client — the same canonical data source the in-process HTTP API surfaces at /api/v1/tencent/resources/{type}.

Why

clanker-cloud is building Tencent provider support (clankercloud/clanker-cloud PR #470 onward) and needs --json output to shell out cleanly. Without this flag the backend would have to either parse tabwriter output (brittle) or spawn clanker server as a managed sidecar (lifecycle complexity that was explicitly considered and rejected). This change is the smallest possible surface to unblock that work.

Behaviour

• clanker tencent list cvm — unchanged (table)
• clanker tencent list cvm --format json — single-region JSON array (matches the HTTP API shape exactly)
• clanker tencent list cvm --format json --all-regions — explicit envelope:

  {
    "regions": [
      {"region": "ap-singapore", "data": [...]},
      {"region": "ap-tokyo", "data": [...]}
    ],
    "errors": [
      {"region": "ap-shanghai-fsi", "error": "..."}
    ]
  }

  Per-region errors so a single regional outage doesn't poison the whole sweep.
• clanker tencent list subnets --format json — returns a guidance error pointing the user at vpc --format json (subnets are embedded in the VPC summary, no dedicated JSON method).
• clanker tencent list cvm --format xml — clear error: unsupported --format "xml" (use 'table' or 'json').

Test plan

• go test ./internal/tencent/ -run TestEmitTypedJSON — passes (dispatch + subnets guidance)
• go vet ./... clean
• gofmt -s -l . empty
• clanker tencent list cvm --format json --help shows the new flag in the rendered help
• clanker tencent list cvm (no format) still emits the table
• clanker tencent list cvm --format xml errors out with a clear message (once creds are present — the credential check is intentionally first)
• Live smoke against real Tencent creds — JSON output should match the existing HTTP API response shape byte-for-byte (both call the same JSONX methods)

Notes for reviewers

• All new logic is in internal/tencent/list_json.go (clean dispatch table). The existing static_commands.go change is two flag additions plus a single short-circuit at the top of the list RunE.
• Backward-compatible: every existing CLI invocation behaves identically.
• The format-validation happens after credential resolution by design — credentials are a more fundamental error to surface first, so a typo in --format returns the credential message rather than a format-validation message when both are wrong.