fix: throw TypeError for non-JSON-serializable values in c.json()

[veeceey]

Fixes #2343

When c.json(undefined) is called, JSON.stringify(undefined) returns undefined (not a string), which leads to inconsistent behavior across runtimes - Bun returns an empty string "", Cloudflare returns "undefined", and Node throws an error.

This follows the same approach discussed in the issue thread, matching Deno's implementation: check the result of JSON.stringify and throw a TypeError if the value isn't serializable. This way the behavior is consistent across all runtimes instead of silently returning wrong responses.

Also moved the fast path to use new Response(body, ...) instead of Response.json(object) so the serialization check happens before sending the response regardless of which code path is taken.

Added a test to verify the behavior.

[veeceey]

Test Results

Ran the full test suite - all context tests pass including the new test:

✓ |main| src/context.test.ts (58 tests) 17ms
  Tests  58 passed (58)

Full suite:

Test Files  1 failed | 143 passed (144)
     Tests  4 failed | 4089 passed | 33 skipped (4126)

The 4 pre-existing failures are in runtime-tests/node/index.test.ts (stream abort handler tests) and unrelated to this change.

Manual verification:

// Before: JSON.stringify(undefined) → undefined → new Response(undefined) → "" (Bun) / "undefined" (CF)
// After: throws TypeError("Value is not JSON serializable")

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.48%. Comparing base (2de30d7) to head (d5fb0e9).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4756   +/-   ##
=======================================
  Coverage   91.48%   91.48%           
=======================================
  Files         177      177           
  Lines       11556    11556           
  Branches     3357     3359    +2     
=======================================
  Hits        10572    10572           
  Misses        983      983           
  Partials        1        1           

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[yusukebe]

@veeceey

Thank you for the PR. If this issue is introduced by #4707, #4757 fixed the problem.