feat(billing): add usage.get method, update usage.paygo endpoint

[vaishakdinesh]

Sync billing resource from staging-next

Brings in the previously-deferred billing resource changes. SDK call site is
unchanged for existing callers; the deferred "breaking" label was driven by an
internal endpoint URL change, not by an SDK surface change.

Source: origin/staging-next (billing-only checkout)
Base: origin/next

Local Validation (all gates pass)

Gate	Result
npx tsc --noEmit	pass
eslint .	pass
./scripts/detect-breaking-changes origin/next	pass (exit 0)

What changed

SDK call site: unchanged.

// Same call, same params, same return type
client.billing.usage.paygo({ account_id: '...' });

New (additive):

• client.billing.usage.get({ account_id, from?, to?, metric? }) -- new endpoint
  GET /accounts/{account_id}/billable/usage returning a FinOps FOCUS v1.3
  cost-and-usage dataset (UsageGetResponse).
• UsagePaygoResponse.Record.ServiceFamilyName? -- new optional field.

Internal:

• client.billing.usage.paygo() now calls
  GET /accounts/{account_id}/paygo-usage instead of
  GET /accounts/{account_id}/billing/usage/paygo. Method signature, params,
  and return type are unchanged.

Why this is not user-breaking

• detect-breaking-changes (which lints the OLD test files against the NEW
  SDK) passes with exit 0 -- the SDK-level API contract is preserved.
• The change-analyzer (scripts/common/summarize-diff.ts) flags the
  endpoint URL change as "breaking" because it tracks underlying API path
  changes, but that's a server-side concern (the API team must keep the old
  path alive or redirect during transition), not a code change anyone using
  the SDK needs to make.

What's still deferred

The other 5 previously-deferred resources still have SDK-surface breaking
changes and remain out of this PR: addressing, api-gateway, cache,
email-security, realtime-kit.