fix(firestore-send-email): make attachment validation more lenient and robust

[cabljac]

Fixes #2550 - Email delivery failing with "Expected array, received object" Zod validation error.

The attachmentsSchema now uses z.preprocess() to normalize attachment inputs before validation:

• Single attachment objects are automatically wrapped in an array
• null values are treated as "no attachments"
• Empty attachment objects are filtered out

Also replaced direct .parse() calls with .safeParse() for better error handling.

Test plan

• Existing tests updated to reflect new lenient behavior
• Added test for single attachment object normalization
• All validation and prepare-payload tests pass