Raise flatbuffers verifier max_depth for pipeline deserialization

[alexreinking]

Summary

#9395 added flatbuffer verification before deserializing a pipeline, but used the verifier's default max_depth of 64. Each Stmt/Expr node serializes as one flatbuffer table, so any pipeline with a moderately deep expression nests past that and gets rejected as "malformed" even though it's valid. This is what broke the serialization round-trip CI bot (arm-64 / coverage, which only runs the roundtrip flag on push to main, not on PRs) and led to the revert in #9426.

This is a fix-forward: raise max_depth to 1000 at both verification call sites instead of leaving verification out entirely.

• Reproduced the exact CI failures from the arm-64 / coverage job logs: correctness_align_bounds, correctness_code_explosion, correctness_integer_powers, correctness_loop_carry, correctness_lots_of_loop_invariants, correctness_many_inlined_selects — all build a single Expr nested past depth 64 (e.g. summing 100+ loop invariants into one Add tree).
• 1000 is a judgment call, not a derived constant: comfortably above the deepest failing test (~200-400) and the coverage bot doesn't exercise apps/performance tests, so the deepest real pipelines are untested against this bound. It's also well within safe recursion depth for deserialize_stmt/deserialize_expr, which recurse to a depth matching the buffer's nesting.
• Verified a corrupted buffer is still rejected, so this doesn't disable the ASAN-motivated protection verify serialized buffer before accessing it in the deserializer #9395 was added for — it only widens what counts as structurally valid.

Test plan

• Reproduced all 6 originally-failing correctness tests locally under WITH_SERIALIZATION_JIT_ROUNDTRIP_TESTING, confirmed they pass with this fix
• Added test/correctness/serialization.cpp: round-trips a pipeline with an Expr nested past the old default depth, and confirms a corrupted buffer still throws
• Confirmed the new test fails without the fix and passes with it
• correctness_streaming and correctness_generator_cache (the existing tests that exercise real serialize/deserialize round-trips) still pass
• pre-commit run passes on all changed files

Co-authored-by: nashit hayat nashit@bugqore.com
Co-authored-by: Claude Sonnet 5 noreply@anthropic.com

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.16%. Comparing base (0537858) to head (178c03e).

Files with missing lines	Patch %	Lines
src/Deserialization.cpp	50.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9427      +/-   ##
==========================================
+ Coverage   70.02%   70.16%   +0.14%     
==========================================
  Files         261      261              
  Lines       79393    79403      +10     
  Branches    19359    19361       +2     
==========================================
+ Hits        55593    55713     +120     
+ Misses      17915    17892      -23     
+ Partials     5885     5798      -87     

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