fix(tree): use url.PathUnescape for path parameters

[baltasarblanco]

Pull Request Checklist

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• Open your pull request against the master branch.
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Description

Replace url.QueryUnescape with url.PathUnescape when decoding wildcard parameter values in the radix tree. Per RFC 3986, URL path segments preserve + as a literal character; the + → space convention belongs to application/x-www-form-urlencoded, not to URL paths. This aligns Gin with the upstream httprouter and Go stdlib semantics.

Scope of impact

Affects only configurations using UseEscapedPath or UseRawPath together with UnescapePathValues=true. The default routing path is unchanged because URL.Path is already decoded by net/http upstream of getValue, so unescape is false and neither function is invoked.

Behavior change (called out explicitly)

A URL path param containing a literal + (e.g. /items/a+b) previously decoded to a b; it now correctly decodes to a+b. Percent-encoded + (%2B) still decodes to +. Clients needing a space in a path segment should URL-encode it as %20 (RFC 3986).

Tests

• Two cases in TestUnescapeParameters previously asserted the incorrect + → space behavior and have been updated.
• A new case verifies that %2B still decodes to +.
• Full matrix passes locally: default, -race, -tags=nomsgpack, -tags=go_json, -tags=sonic (with --ldflags="-checklinkname=0").

Fixes #3850

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.37%. Comparing base (3dc1cd6) to head (4f17499).
⚠️ Report is 276 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4674      +/-   ##
==========================================
- Coverage   99.21%   98.37%   -0.84%     
==========================================
  Files          42       48       +6     
  Lines        3182     3138      -44     
==========================================
- Hits         3157     3087      -70     
- Misses         17       42      +25     
- Partials        8        9       +1     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	98.36% <100.00%> (?)
-tags go_json	98.29% <100.00%> (?)
-tags nomsgpack	98.35% <100.00%> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.25	98.37% <100.00%> (?)
go-1.26	98.37% <100.00%> (?)
macos-latest	98.37% <100.00%> (-0.84%)	⬇️
ubuntu-latest	98.37% <100.00%> (-0.84%)	⬇️

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