fix: middleware not matching routes with optional parameters in fs routing

[bartlomieju]

Summary

• Fixed app.use("/api", middleware) not applying to fs routes with optional parameters like routes/api/[[opt]]/endpoint.ts
• The root cause was patternToSegments() splitting on / characters inside {...} groups in URLPattern syntax

When routes/api/[[opt]]/endpoint.ts is converted to the pattern /api{/:opt}?/endpoint, the / inside {/:opt}? was treated as a segment boundary, producing segments ["", "api{", ":opt}?"] instead of ["", "api"]. This caused the route to land in a different segment tree node than the middleware registered at /api.

The fix strips {...}? optional groups before segmenting, so /api{/:opt}?/endpoint produces the same segments as /api/endpoint.

Closes #3545

Test plan

• Added test cases for patternToSegments with {/:opt}? syntax
• All 59 router/segments/app tests pass
• All 22 builder tests pass

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