feat: add --dry-run to emails batch and broadcasts send

[Abhinav-2002]

See discussion: #398

Closes the gap noted in the README:

likely next candidates are emails batch (large JSON files) and
broadcasts send (confirm id + schedule before delivery).

What changed

resend emails batch --file ./emails.json --dry-run

Parses and validates the JSON array, normalises snake_case field aliases
(scheduled_at → scheduledAt, reply_to → replyTo, topic_id → topicId),
and renders any --react-email template. Note: rendering a --react-email template
uses esbuild to bundle and run the .tsx file, which means it executes local code
even during a dry-run.

The command then prints:

{ "dryRun": true, "request": [ ... ] }

No API key required (and it skips verifying key permissions). All existing validation (bad JSON, missing file, attachments
in batch, per-entry type checks) still runs before bail-out — you get the same
error codes as a real send. Note: passing an empty array [] now fails with
invalid_format to enforce non-empty batches during the dry-run, which also applies
to live runs.

resend broadcasts send <id> --dry-run

Echoes back the provided broadcast ID (using the interactive picker if omitted),
then prints:

{ "dryRun": true, "request": { "id": "...", "scheduledAt": "..." } }

No API key required, so the ID is not verified against the API. scheduledAt is
omitted when not set.

Output format

{ "dryRun": true, "request": ... } — identical to emails send --dry-run and
broadcasts create --dry-run.

Testing

Automated (8 new tests, 0 regressions):

emails batch --dry-run:

• No SDK call made
• Works without an API key
• Empty array [] properly errors with invalid_format
• snake_case aliases normalised in printed output
• --react-email renders into request[n].html without sending

broadcasts send --dry-run:

• No SDK call made
• scheduledAt appears in request when --scheduled-at is set; absent when not
• Works without an API key

Pre-existing failures on table.test.ts / spinner.test.ts (Windows terminal
encoding: ASCII fallback + vs Unicode ┌, v vs ✔) are unrelated to this
change — they fail on clean main too.

Manual:

# batch dry-run with delivered@resend.dev / bounced@resend.dev — nothing sent
node dist/cli.cjs emails batch --file ./emails.json --dry-run

# broadcasts send dry-run with fake ID — nothing sent
node dist/cli.cjs broadcasts send fake-id-1234 --scheduled-at "in 2 hours" --dry-run

# no API key — still works
unset RESEND_API_KEY
node dist/cli.cjs emails batch --file ./emails.json --dry-run   # exit 0
node dist/cli.cjs broadcasts send fake-id --dry-run             # exit 0

# bad input — clear errors
node dist/cli.cjs emails batch --file broken.json --dry-run
# => {"error":{"message":"File content is not valid JSON.","code":"invalid_json"}}

Note on AI assistance

I used an AI coding assistant (Antigravity) to help write this. I read every changed
line, ran the full test suite, and verified all the manual behaviour above myself.
The design decisions (where to bail, what to output, how to handle react-email) are
ones I can explain.

---

Summary by cubic

Adds --dry-run to emails batch and broadcasts send so complex payloads can be validated and inspected before a high-impact send.

• Prints { "dryRun": true, "request": ... } without calling the API, matching emails send and broadcasts create, and skips the API key and permission check.
• All existing validation and error codes still apply; --react-email templates are rendered during dry-run, which executes the local .tsx code.
• Empty batch arrays now fail with invalid_format during dry-run, and this also applies to live sends.
• broadcasts send --dry-run echoes the given ID (or prompts for it interactively) along with any scheduledAt.

^{Written for commit f3630d6. Summary will update on new commits.}