GH-51289: [C++] Add iterator-based bit-run traversal

[akashchamp]

Fixes #51289

Rationale for this change

The callback-based bit-run visitors require callers to route early termination through callback status handling. These ranges allow normal iterator control flow, including breaking once a caller has found the run it needs.

What changes are included in this PR?

• Add IterateBitRuns, IterateSetBitRuns, and IterateTwoSetBitRuns as C++20 input ranges.
• Preserve the existing BitRun type and introduce PositionedBitRun for full bit runs so iterator values expose position, length, and set state.
• Cover offsets, null bitmaps, empty inputs, iterator copying and post-increment, and intersections longer than the legacy visitor's internal chunk size.

Are these changes tested?

• clang-format --dry-run --Werror and git diff --check
• Focused arrow-bit-utility-test --gtest_filter='TestSetBitRunReader.Iterate*' (3 tests)
• Full arrow-bit-utility-test (217 tests) and its focused CTest entry
• Manual validation with a standalone C++20 program that compared 3,000 randomized bitmap, offset, length, null-bitmap, and early-break cases against independent bit scans.

Are there any user-facing changes?

No public API changes. This adds internal C++ traversal helpers for callers that need iterator control flow.

AI assistance

Assisted-by: Codex

AI assistance was used for the initial implementation and regression tests. I reviewed the final code, kept the change scoped to the requested API, and manually validated the behavior above.

• GitHub Issue: [C++] Add iterator-based variants of bit-run visitors #51289

[github-actions]

⚠️ GitHub issue #51289 has been automatically assigned in GitHub to PR creator.