perf(context): add fast path to c.json() matching c.text() optimization

[mgcrea]

c.text() already has a fast path (added in an earlier optimization) that returns new Response(text) directly when no status, headers, or finalized state exists. c.json() was missing this optimization and always went through #newResponse(), which unconditionally allocates a Headers object and iterates header entries — even for the simplest c.json({ ... }) call.

This PR adds the same 5-condition fast path to c.json().

What changed

In src/context.ts, the json() method now checks:

!this.#preparedHeaders && !this.#status && !arg && !headers && !this.finalized

When all conditions are met (the common case for simple JSON responses), it creates the Response directly with an inline header object instead of going through #newResponse():

new Response(jsonString, {
  headers: { 'Content-Type': 'application/json' },
})

Why this matters

In the @hono/node-server adapter, this has a compounding effect. The lightweight Response constructor stores inline header objects directly in its cache. When the adapter writes the response, it checks header instanceof Headers — a plain object skips the buildOutgoingHttpHeaders() conversion entirely, while a Headers instance requires iteration and conversion to OutgoingHttpHeaders.

Benchmark results (nodejs)

with corresponding @hono/node-server optimizations, see honojs/node-server#301

Benchmark	Before	After	Change
Ping	55,183	59,697	+8.2%
Query	46,033	52,883	+14.9%
Body	26,439	46,726	+76.7%
Average	42,552	53,102	+24.8%

Body benchmark improvement is primarily from @hono/node-server direct body reading; this PR contributes to the overall gains by eliminating allocations on the response path.

Benchmarked with bun-http-framework-benchmark using bombardier at 500 concurrent connections for 10s per test on Node.js.

---

Thanks for the great lib :)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.44%. Comparing base (69ad885) to head (a32bdde).
⚠️ Report is 16 commits behind head on next.

Additional details and impacted files

@@            Coverage Diff             @@
##             next    #4707      +/-   ##
==========================================
+ Coverage   91.43%   91.44%   +0.01%     
==========================================
  Files         173      173              
  Lines       11382    11424      +42     
  Branches     3302     3319      +17     
==========================================
+ Hits        10407    10447      +40     
- Misses        974      976       +2     
  Partials        1        1              

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[yusukebe]

@mgcrea

This PR is super cool. Especially if adding this fast path, we can use Response.json() as commented in #4707 (comment). If so, it can reduce the bundle size and get it faster.

Sorry! I completely misunderstood. The file size is slightly increased. But it's interesting that we can use Response.json() (#4707 (comment)) with this PR.

I would like to merge this. Can you handle my comments? Or, I'll fix them myself.