Add tests for panic behavior on malformed route patterns

[fheikens]

Summary

Adds test coverage for five existing panic guard paths in route pattern
validation that were not explicitly covered by tests. No production code was changed.

Covered cases:

• Unclosed { brace in route pattern (tree.go:patNextSegment)
• Duplicate parameter keys in pattern (tree.go:patParamKeys) — addresses the TODO at tree_test.go:194
• Invalid regexp in route parameter (tree.go:addChild)
• Route() called with nil function (mux.go:Route)
• Mount() called with nil handler (mux.go:Mount)

Test plan

• All five new tests pass: go test -v -run TestPanic ./...
• Full test suite passes: go test ./...
• No production code modified

[themavik]

Good coverage of router construction panics (unclosed {, duplicate {id}, bad regexp, nil Route/Mount). nit: TestPanicOnInvalidRegexp uses {id:[invalid}—if that ever stops compiling for a different reason, the test name will lie; tighten the pattern if you see flakes.

[fheikens]

Good point — tightened the pattern to {id:(abc} (unclosed group) so the failure is unambiguously from regexp.Compile, not from route pattern parsing. Test name stays accurate.