fix(emails): flatten metrics filter query params

[dielduarte]

Summary

• The public-api's /emails/metrics endpoint now accepts domain_id/email_id as flat query params instead of filter[domain_id]/filter[email_id] (resend/resend-monorepo#7974).
• Updates buildMetricsQuery in src/emails/emails.ts to send the flat params. The public SDK option shape (filter: { domainId, emailId }) is unchanged — only the wire format sent to the API changed.

Test plan

• Updated emails.spec.ts expected request URLs to the flat param format
• vitest run src/emails/emails.spec.ts — 33 passing
• biome check — clean

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Summary by cubic

Flattened /emails/metrics query params to use domain_id and email_id instead of filter[domain_id] and filter[email_id], matching the public API contract. Updated buildMetricsQuery to send the flat params; the SDK option shape (filter: { domainId, emailId }) remains unchanged.

^{Written for commit 2c23260. Summary will update on new commits.}

[klotty]

No issues found across 2 files

Confidence score: 5/5

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

<sub>Auto-approved: Focused fix aligning SDK wire format with the updated /emails/metrics API; the public SDK option shape is unchanged and tests pin the new flat params. No product, security, or operational tradeoff remains for human review.

Re-trigger cubic</sub>