chore: use the API timestamp format in spec fixtures

[felipefreitag]

What

The API returns timestamps in Postgres text format: YYYY-MM-DD HH:MM:SS[.ffffff]+00 (for example 2023-10-06 23:47:56.678+00). Spec fixtures and their assertions showed ISO 8601 for response fields. This PR makes them match the real API output.

• Convert 120 response-fixture timestamps and paired assertions in 24 spec files.
• spec/emails/receiving_spec.rb:113 already used the correct format; the rest of the repo now matches it.

What stays ISO 8601 on purpose

• scheduled_at as a request param (spec/emails_spec.rb, examples/schedule_email.rb). The API accepts ISO there.
• The webhook event payload strings in the signature verification specs. Webhooks emit ISO, and these strings feed the HMAC computation.
• revoked_at in the revoke OAuth grant response. That endpoint returns ISO.

Why

The SDK never parses these values, so there are no runtime changes. But wrong fixtures propagate: new endpoints copy the pattern, and users who read the specs get the wrong contract.

Verification

• bundle exec rake spec: 336 examples, 0 failures.
• bundle exec rubocop: no offenses.

---

Summary by cubic

Updated spec fixtures and assertions to use the API’s Postgres timestamp format so tests match real responses. No runtime changes.

• Refactors
  • Converted 120 timestamps across 24 spec files from ISO 8601 to Postgres text format (YYYY-MM-DD HH:MM:SS[.ffffff]+00).
  • Kept ISO 8601 where the API uses it: scheduled_at request param, webhook event payload strings in signature specs, and revoked_at in the revoke OAuth grant response.

^{Written for commit 037b2c4. Summary will update on new commits.}

[klotty]

No issues found across 24 files

Confidence score: 5/5

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

<sub>Auto-approved: All changes are in spec fixtures, converting 120 timestamps from ISO 8601 to the API's Postgres text format. No runtime logic, schema, or public contract changes. The diff confirms a focused test alignment with no meaningful tradeoffs for human review.

Re-trigger cubic</sub>