net/mail: avoid quadratic behavior in mail address parsing

RFC 5322 domain-literal parsing built the dtext value one character
at a time with string concatenation, resulting in excessive
resource consumption when parsing very large domain-literal values.

Replace with a subslice.

Benchmark not included in this CL because it's too narrow to be
of general ongoing use, but for:

    ParseAddress("alice@[" + strings.Repeat("a", 0x40000) + "]")

goos: darwin
goarch: arm64
pkg: net/mail
cpu: Apple M4 Pro
                │  /tmp/bench.0  │            /tmp/bench.1             │
                │     sec/op     │   sec/op     vs base                │
ParseAddress-14   1987.732m ± 9%   1.524m ± 5%  -99.92% (p=0.000 n=10)

                │   /tmp/bench.0   │             /tmp/bench.1              │
                │       B/op       │     B/op      vs base                 │
ParseAddress-14   33692.767Mi ± 0%   1.282Mi ± 0%  -100.00% (p=0.000 n=10)

                │  /tmp/bench.0  │            /tmp/bench.1            │
                │   allocs/op    │ allocs/op   vs base                │
ParseAddress-14   263711.00 ± 0%   17.00 ± 0%  -99.99% (p=0.000 n=10)

Thanks to Philippe Antoine (Catena cyber) for reporting this issue.

Fixes CVE-2025-61725
Fixes #75680

Change-Id: Id971c2d5b59882bb476e22fceb7e01ec08234bb7
Reviewed-on: https://go-internal-review.googlesource.com/c/go/+/2840
Reviewed-by: Roland Shoemaker <bracewell@google.com>
Reviewed-by: Nicholas Husin <husin@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/709860
Reviewed-by: Carlos Amedee <carlos@golang.org>
TryBot-Bypass: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>