docs: explain cursor limits and POST search bodies

[hadrien]

Problem

• The cursor size cap and its relationship to HTTP transport are undocumented.
• The cursor example does not show structured search parameters in a JSON body.

Solution

• Document the 4096-character cap and oversized input/output behavior.
• Show validated POST body pagination, filtering, and allowlisted ordering with the existing factory.
• Explain explicit dependency arguments and consistent filters and ordering between pages.

Next

• Merge feat: add native forward cursor pagination #164 first, then retarget this documentation follow-up to main.

Bead: fs-rjt

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.79%. Comparing base (29a62ca) to head (26ca774).
⚠️ Report is 1 commits behind head on feat/cursor-pagination.

Additional details and impacted files

@@                   Coverage Diff                   @@
##           feat/cursor-pagination     #165   +/-   ##
=======================================================
  Coverage                   98.79%   98.79%           
=======================================================
  Files                           1        1           
  Lines                         248      248           
  Branches                       30       30           
=======================================================
  Hits                          245      245           
  Misses                          3        3           

Flag	Coverage Δ
fastapi-post121	98.79% <ø> (ø)
fastapi-pre121	98.79% <ø> (ø)
sqlmodel	98.79% <ø> (ø)

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