feat(executor): expose return codec in ApplyRet

[rvagg]

This is part of FIP-0107 which may or may not be accepted, but it's also independent because this is a long-standing TODO. I attempted to find a good way to test this but currently we have no infra that runs a test actor returning a result with a known codec. I judged this trivial enough to just go ahead with without the test.

This is a breaking API change though because of the addition to ApplyRet. I have a follow-up for filecoin-ffi but Forest will need to deal with it separately.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 73.80952% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.58%. Comparing base (8e2f86b) to head (b6052ae).

Files with missing lines	Patch %	Lines
fvm/src/executor/default.rs	75.60%	10 Missing ⚠️
fvm/src/executor/mod.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2264      +/-   ##
==========================================
- Coverage   77.59%   77.58%   -0.02%     
==========================================
  Files         147      147              
  Lines       15851    15865      +14     
==========================================
+ Hits        12300    12309       +9     
- Misses       3551     3556       +5     

Files with missing lines	Coverage Δ
fvm/src/executor/mod.rs	20.58% <0.00%> (-0.63%)	⬇️
fvm/src/executor/default.rs	77.44% <75.60%> (-0.28%)	⬇️

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[Copilot]

Pull request overview

This PR updates the FVM executor API to expose the IPLD codec associated with top-level return data, addressing a long-standing TODO and aligning ApplyRet with existing syscall-level return metadata.

Changes:

• Add a new return_codec: u64 field to ApplyRet (with 0 indicating no return data).
• Plumb the return codec through the default executor by extracting it from the invocation return block and returning it alongside the receipt.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
fvm/src/executor/mod.rs	Extends ApplyRet with a return_codec field and initializes it in prevalidation_fail.
fvm/src/executor/default.rs	Extracts the codec from the invocation return block and propagates it into the produced ApplyRet.

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