docs(zod-validator, standard-validator): document default error response format

[arsalanghogari]

Part 1 of the plan proposed in #2049 (docs first, opt-in RFC 9457 formatter as a possible follow-up).

What this adds

For both @hono/zod-validator and @hono/standard-validator READMEs:

• A Default error response section showing the exact JSON body returned on validation failure when no hook is provided (captured from the current published packages), with notes that:
  • zod-validator returns Zod's serialized safeParse result, where error.message is a JSON string of the issue array
  • standard-validator echoes the raw request input back in data, which may be undesirable for sensitive values
  • neither format is guaranteed stable, so clients parsing the body should use a hook
• A hook example producing an RFC 9457 Problem Details response with application/problem+json, so the standard format is one copy-paste away

Both example hooks were run against the current packages to confirm they work as shown. Docs only, no behavior changes, so no changeset included.

[changeset-bot]

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Latest commit: 6ae4b1c

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.14%. Comparing base (84984d4) to head (6ae4b1c).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2060   +/-   ##
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  Coverage   92.14%   92.14%           
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  Files         115      115           
  Lines        4113     4113           
  Branches     1072     1072           
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  Hits         3790     3790           
  Misses        287      287           
  Partials       36       36           

Flag	Coverage Δ
oauth-providers	92.55% <ø> (ø)
standard-validator	96.10% <ø> (ø)
ua-blocker	90.00% <ø> (ø)
zod-validator	91.30% <ø> (ø)

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