fix: Restore offline file delivery section, update pre commit hook from Docusaurus to Mintlify build check

[BaiyuScope3]

Summary

This PR restores the "Offline File Delivery (Batch)" section that was removed in PR #203 and updates the pre-push validation to use Mintlify instead of Docusaurus.

• Restored comprehensive format examples for JSONL, CSV, and Parquet formats
• Fixed empty space issue at #offline-file-delivery-batch anchor
• Reordered the Delivery Methods section to make the structure more reasonable - Delivery Methods is now the parent section with Webhook-Based Reporting and Offline-File-Delivery-Based Reporting as subsections
• Updated pre-push hook to validate with Mintlify (broken links and accessibility checks) instead of Docusaurus build
• Aligned validation with production system (Mintlify)
• Added missing fields (notification_type, sequence_number, next_expected_at) to all offline file format examples
• Updated CSV format to use dot notation (by_package.pricing_model, totals.impressions)

Documentation Fixes

• Restored "Offline File Delivery (Batch)" section - Previously removed due to MDX parsing errors
• Fixed tab syntax compatibility - Switched from Docusaurus TabItem to Mintlify <Tab> syntax
• Added comprehensive format examples - JSONL, CSV, and Parquet formats with detailed examples and file naming conventions
• Fixed empty space issue - Resolves the empty section at #offline-file-delivery-batch anchor

Infrastructure Changes

• Updated pre-push hook validation:
  • Removed Docusaurus build check (production uses Mintlify)
  • Added Mintlify broken links validation (fails on broken internal links)
  • Added Mintlify accessibility check (warns on a11y issues, non-blocking)
• Aligned validation with production - Pre-push now validates using the same system as production

Technical Details

Why it was removed before:

• PR fix: restore Mintlify docs configuration and fix MDX parsing error #203 removed the section because Docusaurus TabItem syntax caused MDX parsing errors in Mintlify
• The pre-push hook was running Docusaurus build, which failed on Mintlify-specific syntax

Solution:

• Use Mintlify-native <Tab> components (no imports needed)
• Validate with Mintlify tools instead of Docusaurus build
• Ensures documentation works in production (Mintlify) before pushing

Test Plan

• ✅ All schema validation tests pass
• ✅ TypeScript compilation succeeds
• ✅ Mintlify broken links check passes (no broken links found)
• ✅ Mintlify accessibility check runs (warns on color contrast, non-blocking)
• ✅ 88 MDX files validated successfully
• ✅ Pre-push hook validation passes
• ✅ Section renders correctly in Mintlify (no empty space at anchor)

Files Changed

• docs/media-buy/media-buys/optimization-reporting.mdx - Restored section with Mintlify-compatible tabs (+129 lines)
• .husky/pre-push - Updated validation to use Mintlify instead of Docusaurus