fix(embedded): Remove CSRF requirement for dashboard download API

[Vitor-Avila]

SUMMARY

#29187 introduced a new API endpoint to process the dashboard PDF / PNG download. Since the Download > menu options can be exposed in Embedded, then this new endpoint must accept requests without a CSRF token.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

No UI changes.

TESTING INSTRUCTIONS

1. Launch a dashboard in embedded mode (make sure that chartControls are visible).
2. Trigger a dashboard download.

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.70%. Comparing base (76d897e) to head (bb922ad).
⚠️ Report is 2419 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #29953       +/-   ##
===========================================
+ Coverage   60.48%   83.70%   +23.21%     
===========================================
  Files        1931      527     -1404     
  Lines       76236    38069    -38167     
  Branches     8568        0     -8568     
===========================================
- Hits        46114    31866    -14248     
+ Misses      28017     6203    -21814     
+ Partials     2105        0     -2105     

Flag	Coverage Δ
hive	49.01% <ø> (-0.14%)	⬇️
javascript	?
mysql	76.78% <ø> (?)
postgres	76.86% <ø> (?)
presto	53.56% <ø> (-0.25%)	⬇️
python	83.70% <ø> (+20.20%)	⬆️
sqlite	76.34% <ø> (?)
unit	60.36% <ø> (+2.73%)	⬆️

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