Add flat_map_ok iterator adaptor

[veeceey]

Adds a flat_map_ok adaptor that applies a function to Ok values and flattens the result, passing Err values through unchanged. Basically a combined .map_ok(f).flatten_ok() in a single step.

The implementation follows the same pattern as the existing flatten_ok adaptor, with support for:

• DoubleEndedIterator (when the inner iterator supports it)
• FusedIterator
• Clone / Debug
• fold / rfold optimizations

Example usage:

let input = vec![Ok(2i32), Err(false), Ok(3i32)];
let it = input.into_iter().flat_map_ok(|i| 0..i);
// yields: Ok(0), Ok(1), Err(false), Ok(0), Ok(1), Ok(2)

Closes #528

[veeceey]

Test results:

running 8 tests
test flat_map_ok_all_errors ... ok
test flat_map_ok_collect_ok_reverse ... ok
test flat_map_ok_collect_ok_forward ... ok
test flat_map_ok_collect_mixed ... ok
test flat_map_ok_empty_results ... ok
test flat_map_ok_equivalence_with_map_ok_flatten_ok ... ok
test flat_map_ok_mixed_forward ... ok
test flat_map_ok_mixed_reverse ... ok

test result: ok. 8 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

Full test suite also passes (202 tests).

[codecov]

Codecov Report

❌ Patch coverage is 61.53846% with 40 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.25%. Comparing base (6814180) to head (d092fd7).
⚠️ Report is 178 commits behind head on master.

Files with missing lines	Patch %	Lines
src/flat_map_ok.rs	56.98%	40 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1087      +/-   ##
==========================================
- Coverage   94.38%   93.25%   -1.14%     
==========================================
  Files          48       52       +4     
  Lines        6665     6571      -94     
==========================================
- Hits         6291     6128     -163     
- Misses        374      443      +69     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.