fix: validate email format for optional email fields

[hbjORbj]

What does this PR do?

This PR fixes a validation gap where phone numbers could be entered in optional email fields without any format validation, causing 500 errors during iCal generation and silent email sending failures.

Related to #27217 which fixes the error handling for this issue.

The Fix

In getBookingResponsesSchema.ts, email format validation now runs whenever a non-empty value is provided, even if the field is optional. This follows the same pattern already used for phone field validation:

const needsValidation =
  checkOptional || bookingField.required || (value && String(value).trim() !== "");

Since getBookingResponsesSchema is shared between frontend (useBookingForm.ts) and backend, this fix validates on both sides.

Context

When an event type has the email field configured as optional (phone-only bookings), users could enter a phone number in the email field. Neither frontend nor backend validated the email format for optional fields, which later caused:

• Initial booking: Organizer doesn't receive confirmation email (fails silently)
• Reschedule: Returns 500 error to user (booking succeeds but user sees error)

Tests Added

1. "should validate email format even when email field is optional" - Creates a phone-only event type with optional email, enters a phone number in the email field, and expects a validation error.

2. "should show validation error when phone number is entered in required email field" - Uses a default event type with required email field.

Both tests should now PASS with the validation fix.

Mandatory Tasks (DO NOT REMOVE)

• I have self-reviewed the code (A decent size PR without self-review might be rejected).
• I have updated the developer docs in /docs if this PR makes changes that would require a documentation change. N/A - validation fix only.
• I confirm automated tests are in place that prove my fix is effective or that my feature works.

How should this be tested?

Run the e2e test:

PLAYWRIGHT_HEADLESS=1 yarn e2e apps/web/playwright/email-validation.e2e.ts

Expected results: Both tests should PASS.

Manual testing:

1. Create a phone-only event type (email field optional, phone required)
2. Book the event and enter a phone number in the email field
3. Should see validation error "email_validation_error" preventing submission

Human Review Checklist

• Verify empty optional email fields still work (no validation error when left empty)
• Confirm the validation pattern matches the existing phone field validation logic
• Check that e2e tests pass in CI

Checklist

• My code follows the style guidelines of this project
• I have checked if my changes generate no new warnings

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Link to Devin run: https://app.devin.ai/sessions/cbaf3621d7f143688ae0ad35e196096a
Requested by: @hbjORbj

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E2E results are ready!

• Workflow #101063.1 latest results