perf: add fast paths for cleanPath

[james-yusuke]

Summary

This PR adds fast paths to cleanPath for common already-simple or near-simple paths:

• single clean relative path segments, e.g. abc
• paths with repeated leading slashes followed by a single clean segment, e.g. //abc
• clean absolute paths ending with /segment/.., e.g. /abc/b/..

These cases can be normalized without going through the full buffer-based path cleaning loop.

Benchmarks

Before:

BenchmarkPathCleanLong-10    ~4.02 ms/op    3.22 MB/op    4683 allocs/op

After:

BenchmarkPathCleanLong-10    ~2.63 ms/op    0.81 MB/op    1234 allocs/op

BenchmarkPathClean also reduced allocations from 17 to 15 allocs/op in local runs.

Tests

go test ./...
go test -run='TestPathClean|TestRemoveRepeatedChar'
go test -run=^$ -bench='BenchmarkPathClean(Long)?$' -benchmem -count=5

Checklist

• Open your pull request against the master branch.
• All tests pass in available continuous integration systems (e.g., GitHub Actions).
• Tests are added or modified as needed to cover code changes.
• If the pull request introduces a new feature, the feature is documented in the docs/doc.md.

This PR is a performance optimization and does not introduce a new user-facing feature, so no documentation update is needed.

[codecov]

Codecov Report

❌ Patch coverage is 91.48936% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.04%. Comparing base (3dc1cd6) to head (7595ce6).
⚠️ Report is 289 commits behind head on master.

Files with missing lines	Patch %	Lines
path.go	91.48%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4735      +/-   ##
==========================================
- Coverage   99.21%   98.04%   -1.18%     
==========================================
  Files          42       48       +6     
  Lines        3182     3219      +37     
==========================================
- Hits         3157     3156       -1     
- Misses         17       47      +30     
- Partials        8       16       +8     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	98.02% <91.48%> (?)
-tags go_json	97.96% <91.48%> (?)
-tags nomsgpack	98.01% <91.48%> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.25	98.04% <91.48%> (?)
go-1.26	98.04% <91.48%> (?)
macos-latest	98.04% <91.48%> (-1.18%)	⬇️
ubuntu-latest	98.04% <91.48%> (-1.18%)	⬇️

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