feat(tencent): add --format json to cost by-product and cost top

[rafeegnash]

Summary

Mirrors PR #179 (which added --format json to tencent list) — adds the same flag to tencent cost by-product and tencent cost top. Default behaviour is unchanged (tabwriter output); JSON mode is opt-in.

Why

clanker-cloud is wiring Tencent into the Cost Explorer (Phase d). Without --format json on these subcommands the cost.Provider implementation would have to parse tabwriter output, which is brittle and loses the precision of Tencent's string-encoded decimal amounts.

Wire shapes

// `tencent cost by-product --format json`
{
  "month": "2026-05",
  "items": [
    {"product": "CVM", "real_cost": "12.50", "cash": "10.00", "incentive": "0.00", "voucher": "2.50", "pct": "62.5"}
  ],
  "total": 20.0
}

// `tencent cost top --format json`
{
  "month": "2026-05",
  "top": 5,
  "items": [
    {"product": "CVM", "resource_id": "ins-1", "name": "web-a", "region": "ap-singapore", "pay_mode": "PREPAID", "action": "renew", "cost": "12.50"}
  ]
}

Internals

• listBillByProduct / listBillResourceTop split into buildXReport (typed struct) + writeXTable (preserved text output). Single entry point dispatches by --format.
• New types BillByProductItem / BillByProductReport / BillTopResource / BillTopResourceReport are exported so the cloud backend can consume them directly via JSON.

Test plan

• go test ./internal/tencent/ -run "BillByProduct|BillTopResource" — JSON round-trip tests for both shapes
• go vet ./..., gofmt -s -l . clean
• Help text shows the new flag on both subcommands
• Live smoke with real Tencent creds — JSON output matches the table output byte-for-byte field-wise