fix: deduplicate HTTP methods in 405 Allow header

[nathannewyen]

Summary

• Fixes duplicate HTTP methods appearing in the Allow header of 405 responses
• When multiple overlapping wildcard routes match a path, each matching node was adding its methods to the Allow header, causing duplicates like Allow: POST POST POST POST
• This fix uses a map to track seen methods and only adds each unique method once

Reproduction

As described in the issue, when routes overlap due to wildcards:

r.Post("/article/1-2-3", handler)
r.Post("/article/{a}", handler)
r.Post("/article/{b}-{c}", handler)
r.Post("/article/{b}-{c}-{d}", handler)

A GET request to /article/1-2-3 previously returned:

Allow: POST POST POST POST

Now correctly returns:

Allow: POST

Test plan

• Added TestMethodNotAllowedDuplicateMethods test case
• All existing tests pass

Fixes #996