fix(substrait): Preserve pushed-down table scan offsets and limits

[viirya]

Which issue does this PR close?

Closes #26034.

Rationale for this change

Substrait roundtripping loses pushed-down TableScan offsets and limits when no parent Limit exists. A Projection between the scan and a parent Limit also bypasses the existing offset compensation, changing query results.

What changes are included in this PR?

• Encode a scan's skip/fetch in a FetchRel wrapping its ReadRel.
• Serialize each parent Limit's own offset without adding the scan offset again.
• Add execution-based roundtrip coverage for 96 combinations of scan offsets, fetch counts, optional projections, and parent limits, including zero limits and offsets beyond the input.

What is the testing strategy for this PR?

Verified the offset-only regression against upstream 6047791: the original scan returns [2, 3, 4, 5], while the roundtripped scan incorrectly returns all six rows. The new regression test passes with the fix.

Passed:

• cargo fmt --all
• cargo clippy --all-targets --all-features -- -D warnings
• All Substrait integration tests: 226 passed, 6 ignored.
• Required extended workspace tests: 12,387 passed, 0 failed, 8 ignored; all 526 SQLLogicTests files completed.
• Contributor lint suite through formatting, its feature-specific Clippy check, TOML formatting, license headers, typos, documentation formatting and generation, workflow checks, large-file checks, Markdown links, and security audit.
• Rust documentation check.

No relevant local Substrait producer benchmark is defined.

Are there any user-facing changes?

Substrait roundtrips preserve pushed-down table-scan offsets and limits, including across projections and nested limits. No public API changes.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 93.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.66%. Comparing base (6047791) to head (0bbad2d).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...ubstrait/src/logical_plan/producer/rel/read_rel.rs	92.30%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #26035      +/-   ##
==========================================
+ Coverage   82.64%   82.66%   +0.01%     
==========================================
  Files        1147     1147              
  Lines      445883   446375     +492     
  Branches   445883   446375     +492     
==========================================
+ Hits       368513   368984     +471     
- Misses      54982    54999      +17     
- Partials    22388    22392       +4     

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