docs(transmissions): document test-send endpoint

[danielfsousa]

Summary

Documents the new SparkPost test-send endpoint (POST /v1/transmissions/test-send) in content/api/transmissions.apib.

The endpoint lets a user send a test email of a stored template (draft or published version) to up to 5 recipients — a scoped alternative to creating a full transmission, built for the template-editing workflow.

What was added

• New resource section ## Send a Test Email [/v1/transmissions/test-send] with a ### Send a Test Email [POST] action, placed after the RFC822 content section and before Scheduled Transmissions.
• Access/availability info Banner: requires the templates/test-send grant (held by the Templates, Developer, and Subaccount Developer roles, and custom users with the Templates policy; full Transmissions: Read/Write grant holders can also call it). Web-app sessions only for now — cannot be assigned to API keys yet.
• Request body Data Structure (template_id, draft, recipients as plain string array, substitution_data, metadata, options), with the option restrictions (skip_suppression/start_time rejected, sandbox ignored/auto).
• Key behaviors: test sends are real messages (count toward limits/usage, generate events, respect suppression), automatic sandbox based on the template From domain, and a 6 req/min per-IP rate limit (429).
• Full 422 validation error table, plus + Request / + Response 200 / + Response 422 examples.
• One-line pointer to the new endpoint in the Group intro.

Context

• Design record: https://github.com/SparkPost/access/issues/122#issuecomment-4906928681
• Implementation PRs:
  • Endpoint: https://github.com/SparkPost/transmissions-api/pull/465
  • Grant: https://github.com/SparkPost/access/pull/125
  • Routing + rate limit: https://github.com/SparkPost/public-api-proxy/pull/156
• Access design issue: https://github.com/SparkPost/access/issues/122

Merge timing

These docs should merge only once the feature is fully rolled out — transmissions-api#465 and public-api-proxy#156 deployed, and the access grant (access#125) released.

Validation

Parsed content/api/transmissions.apib with the repo's fury + fury-adapter-apib-parser toolchain (same as the gatsby-transformer-api-blueprint plugin): 0 errors, and the identical warning set as the pre-change baseline (no new warnings introduced). Full Gatsby build not run (too heavy for a content-only change).