fix(macros): support prefix/suffix in TypedPath segments

[dihannahdi]

Motivation

The TypedPath derive macro currently requires each path segment to be entirely a capture (/{id}) or entirely static (/users). This means routes with mixed segments—where a capture is embedded within static text—are rejected by the macro even though the underlying matchit router fully supports them.

Common real-world patterns that fail today:

#[derive(TypedPath, Deserialize)]
#[typed_path("/@{username}")]       // prefix capture
struct AtUsername { username: String }

#[derive(TypedPath, Deserialize)]
#[typed_path("/avatars/{id}.png")]   // suffix capture
struct AvatarPath { id: u32 }

#[derive(TypedPath, Deserialize)]
#[typed_path("/files/{name}.tar.gz")] // prefix + suffix
struct FilePath { name: String }

These all produce a compile error because parse_path() only recognises segments that start with { and end with }.

Closes #3681

Solution

Add a CaptureWithAffix variant to the internal Segment enum:

CaptureWithAffix {
    prefix: String,   // text before `{`
    capture: String,   // the capture name
    suffix: String,    // text after `}`
    span: Span,
}

parse_path() now scans each segment for {…} anywhere within it. When a capture is found with surrounding static text, the new variant is emitted. All downstream functions (format_str_from_path, captures_from_path, field expansion, unit-struct validation) are updated to handle it.

Escaped braces ({{, }}) are left alone as static segments—matching the existing behaviour.

Testing

• New pass test (prefix_suffix_capture.rs) covering named structs, tuple structs, and paths with prefix-only, suffix-only, and prefix+suffix captures.
• All existing tests continue to pass (cargo test -p axum-macros — 28/28 passing).