perf: optimise appendCompact usage

[dannykopping]

Skip appendCompact calls for MarshalJSON output in the internal JSON encoder. The appendCompact function scans every byte to validate and compact JSON, which is O(n) per nested MarshalJSON call. For deeply nested structures, this becomes a significant bottleneck as the same bytes get re-scanned at each nesting level.

There's no need to do HTML escaping again; shims.EscapeHTMLByDefault = true and all MarshalJSON implementations use Marshal() internally, which already does HTML escaping.

For our use of this lib, this func was the biggest CPU hog when encoding JSON payloads at scale.

This is safe because MarshalJSON implementations (both in this SDK and standard library) already produce valid, compact JSON - running appendCompact on the output is redundant work.

goos: linux
goarch: amd64
pkg: github.com/anthropics/anthropic-sdk-go/internal/encoding/json
cpu: AMD Ryzen 9 7900 12-Core Processor             
                                   │  /tmp/old   │               /tmp/new               │
                                   │   sec/op    │    sec/op     vs base                │
MarshalNestedMarshalJSON-24          5.577µ ± 9%   2.660µ ± 13%  -52.31% (p=0.000 n=10)
MarshalSliceOfNestedMarshalJSON-24   301.5µ ± 6%   137.1µ ±  9%  -54.54% (p=0.000 n=10)
geomean                              41.01µ        19.09µ        -53.44%

                                   │   /tmp/old   │              /tmp/new               │
                                   │     B/op     │     B/op      vs base               │
MarshalNestedMarshalJSON-24            914.0 ± 0%     913.0 ± 0%  -0.11% (p=0.000 n=10)
MarshalSliceOfNestedMarshalJSON-24   49.41Ki ± 0%   49.28Ki ± 0%  -0.26% (p=0.000 n=10)
geomean                              6.641Ki        6.629Ki       -0.19%

                                   │  /tmp/old  │              /tmp/new               │
                                   │ allocs/op  │ allocs/op   vs base                 │
MarshalNestedMarshalJSON-24          12.00 ± 0%   12.00 ± 0%       ~ (p=1.000 n=10) ¹
MarshalSliceOfNestedMarshalJSON-24   552.0 ± 0%   552.0 ± 0%       ~ (p=1.000 n=10) ¹
geomean                              81.39        81.39       +0.00%
¹ all samples are equal

For our project, under scale testing this fix reduced CPU usage by ~36%.

I enhanced test coverage and added benchmarks.

Disclaimer: produced alongside Opus 4.5.