fix: reset skipped-nodes stack on getValue entry to prevent slice overflow panic [#4818]

[waterWang]

Fixes #4818

Root cause

With HandleMethodNotAllowed = true, handleHTTPRequest calls getValue
once per method tree in the HandleMethodNotAllowed loop, reusing the
same c.skippedNodes stack from the pooled Context. getValue pushes
entries onto the stack via a raw reslice ([:index+1]) that cannot grow
past engine.maxSections. Without resetting the stack on entry, residue
from a previous tree accumulates until the next reslice exceeds capacity
and panics with "slice bounds out of range".

Fix

Reset *skippedNodes = (*skippedNodes)[:0] at the top of getValue so
every call starts with an empty stack. This is the single point that
covers all callers, including the HandleMethodNotAllowed loop.

Tests

• TestIssue4818_skippedNodesOverflow_Panic: original repro from the
  issue — POST to a route that causes skippedNodes residue to exceed
  maxSections capacity. Confirms no panic and 404 (correct).
• TestIssue4818_MethodNotAllowedStillWorks: PATCH a path that only
  has a GET route, confirming the HandleMethodNotAllowed loop still
  produces a correct 405 with Allow header.
• TestIssue4818_MethodNotAllowedManyTrees: stress test with 9 method
  trees targeting a single GET-only path, verifying no overflow.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.32%. Comparing base (3dc1cd6) to head (b261e00).
⚠️ Report is 292 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4819      +/-   ##
==========================================
- Coverage   99.21%   98.32%   -0.89%     
==========================================
  Files          42       48       +6     
  Lines        3182     3173       -9     
==========================================
- Hits         3157     3120      -37     
- Misses         17       43      +26     
- Partials        8       10       +2     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	98.31% <100.00%> (?)
-tags go_json	98.25% <100.00%> (?)
-tags nomsgpack	98.30% <100.00%> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.25	98.32% <100.00%> (?)
go-1.26	98.32% <100.00%> (?)
macos-latest	98.32% <100.00%> (-0.89%)	⬇️
ubuntu-latest	98.32% <100.00%> (-0.89%)	⬇️

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