Fix inline snapshot corruption with multiple snapshots in with_settings!

[max-sixty]

Summary

Fixes #857

When find_snapshot_macro(line) was called for a line inside a macro like with_settings!, the scan_nested_macros function would scan all nested tokens and find all @"..." patterns. Since try_extract_snapshot always overwrote the result, the last snapshot found would win regardless of which line we were searching for.

This caused:

1. All searches for lines inside with_settings! returned the same (last) snapshot position
2. The duplicate detection logic discarded all but the first pending snapshot
3. The first pending snapshot's content got written to the last snapshot's position

Example from the issue:

insta::with_settings!({filters => vec![...]}, {
    assert_snapshot!(":12345\n\nabc", @"");  // line 10
    assert_snapshot!(":12345\n\nabc", @"");  // line 11
    assert_snapshot!("", @"");               // line 12
    assert_snapshot!("", @"");               // line 13 - incorrectly got content from line 10!
});

The Fix

Instead of filtering during scanning, we now:

1. Collect all snapshots with their macro boundaries (start/end line)
2. Filter at the end to find the one whose macro span contains the target line

This approach is more general because:

• The scanning logic collects everything uniformly
• The line-matching logic is centralized in one place
• It works at any nesting depth

Test plan

• All existing tests pass (111 tests)
• Added regression test test_find_snapshot_macro_multiple_in_with_settings
• Manually verified with the reproduction case from the issue

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