fix(ci): handle old/new workspace paths in op-reth-compact-codec job

[devin-ai-integration]

Summary

The op-reth-compact-codec CI job checks out origin/develop as a base to generate compact test vectors for backward compatibility testing. After the workspace unification in #19034 (moved op-reth/ → rust/op-reth/), the hardcoded manifest path rust/op-reth/bin/Cargo.toml doesn't exist on older develop commits, causing the job to fail with manifest path does not exist.

This adds path detection so the "generate vectors from base" step works regardless of whether the checked-out base has the old (op-reth/) or new (rust/op-reth/) layout.

The "read vectors" step (which runs on the PR branch) is left unchanged since all future branches will use the new path.

Review & Testing Checklist for Human

• Verify the path detection logic correctly covers both old (op-reth/bin/Cargo.toml) and new (rust/op-reth/bin/Cargo.toml) layouts
• Confirm the "Read compact vectors on PR branch" step (line 761, unchanged) does not need similar treatment — it runs on the PR branch which should always have the new path
• Trigger or wait for the op-reth-compact-codec CI job on this PR to confirm the fix works end-to-end (this cannot be tested locally)

Notes

• Link to Devin run
• Requested by: @smartcontracts

[devin-ai-integration]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.0%. Comparing base (48a7a09) to head (c2bb6f2).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #19124      +/-   ##
===========================================
+ Coverage     75.8%    76.0%    +0.2%     
===========================================
  Files          700      188     -512     
  Lines        71986    10946   -61040     
===========================================
- Hits         54574     8327   -46247     
+ Misses       17266     2475   -14791     
+ Partials       146      144       -2     

Flag	Coverage Δ
cannon-go-tests-64	66.4% <ø> (-0.9%)	⬇️
contracts-bedrock-tests	81.6% <ø> (ø)
unit	?

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see 517 files with indirect coverage changes

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[sebastianst]

Wondering if we really need this. The next PR that runs this check will already have a git base after the migration to the rust/ folder.