fix: increase DefaultBodyLimit to prevent large payload failures

[AlexD10S]

When attempting to register a parachain using the wallet to sign the transaction, an error appeared in the frontend:

Error: Unable to submit. Is the server closed?

This was caused by the request body limit being too low, resulting in a 413 (Payload Too Large) error.

This PR increases DefaultBodyLimit to handle larger payloads and adds a test to verify large transaction support. The 15MB limit is a bit arbitrary but ensures stability, providing enough room for 5MB transactions while accounting for potential encoding overhead.

Resource: https://stackoverflow.com/questions/75205271/rust-axum-multipart-length-limit-exceeded.

[codecov]

Codecov Report

Attention: Patch coverage is 96.55172% with 1 line in your changes missing coverage. Please review.

Project coverage is 75.16%. Comparing base (2e03233) to head (7d79720).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/pop-cli/src/wallet_integration.rs	96.55%	0 Missing and 1 partial ⚠️

@@            Coverage Diff             @@
##             main     #409      +/-   ##
==========================================
+ Coverage   75.11%   75.16%   +0.05%     
==========================================
  Files          63       63              
  Lines       13842    13870      +28     
  Branches    13842    13870      +28     
==========================================
+ Hits        10398    10426      +28     
+ Misses       2118     2117       -1     
- Partials     1326     1327       +1     

Files with missing lines	Coverage Δ
crates/pop-cli/src/wallet_integration.rs	96.15% <96.55%> (+0.01%)	⬆️

... and 1 file with indirect coverage changes

[peterwht]

Looks good! Nice fix.

If you have the time, might be easy enough to add a failing test that is 15.1 MB to trigger the error. Not a blocker though

[AlexD10S]

Looks good! Nice fix.

If you have the time, might be easy enough to add a failing test that is 15.1 MB to trigger the error. Not a blocker though

Thanks, it make sense to have that test. Added in the commit 7d79720