feat(usage): add support for the GET /usage endpoint

[dielduarte]

Summary

• Adds resend.Usage.get() / resend.Usage.get_async() for account-level usage/quotas (emails, contacts, segments, broadcasts, ai_credits, automation_runs, domains, rate_limit).
• Live-tested against two real accounts — correct, distinct data returned for each, matching the spec exactly.

Part of the /usage GA rollout (DEV-2183). Reference implementation: resend-node#1111.

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Summary by cubic

Adds resend.Usage.get() and resend.Usage.get_async() to fetch account-level usage and quota data for emails, contacts, segments, broadcasts, AI credits, automation runs, domains, and rate limit.

Rollout and testing

• Part of the /usage GA rollout (DEV-2183), mirroring the Node.js reference implementation.
• Includes sync and async unit tests plus runnable examples, and was live-tested against two accounts with distinct responses.
• Bumps the package version to 2.48.0.

^{Written for commit de69d32. Summary will update on new commits.}

[cubic-dev-ai]

Review completed against the latest diff

<sub>Reply with feedback, questions, or to request a fix.

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[cubic-dev-ai]

All reported issues were addressed across 7 files

<sub>Requires human review: Auto-approval blocked because this review re-detected 2 unresolved issues already reported by Cubic.

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[cubic-dev-ai]

0 issues found across 2 files (changes from recent commits).

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

<sub>Requires human review: Adds new public Usage API (GET /usage) client with sync/async methods, types, tests, and examples. Because this expands the SDK's public API surface, human sign-off is required for the GA rollout.

Re-trigger cubic</sub>

[cubic-dev-ai]

0 issues found across 1 file (changes from recent commits).

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

<sub>Requires human review: Adds a new public Usage API client (GET /usage) with sync/async methods, types, tests, and examples. Expands the SDK's public contract, requiring product/API design sign-off.

Re-trigger cubic</sub>