perf: optimize hex decoding in json (1.8x faster in binary-heavy)

[Weijun-H]

Which issue does this PR close?

• Closes #NNN.

Rationale for this change

Improve JSON binary decoding performance by avoiding per-value allocations and enabling direct hex decoding into builders.

What changes are included in this PR?

Optimized binary hex decoding paths to reduce allocations and improve throughput.

decode_binary_hex_json  time:   [3.6780 ms 3.6953 ms 3.7150 ms]
                        change: [−61.051% −60.818% −60.565%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 8 outliers among 100 measurements (8.00%)
  5 (5.00%) high mild
  3 (3.00%) high severe

decode_fixed_binary_hex_json
                        time:   [4.0404 ms 4.1400 ms 4.2901 ms]
                        change: [−56.149% −55.040% −53.330%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 19 outliers among 100 measurements (19.00%)
  7 (7.00%) high mild
  12 (12.00%) high severe

decode_binary_view_hex_json
                        time:   [4.3731 ms 4.4242 ms 4.4767 ms]
                        change: [−53.305% −52.771% −52.239%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 1 outliers among 100 measurements (1.00%)
  1 (1.00%) high mild

Are these changes tested?

Yes

Are there any user-facing changes?

No