fix: Make the default backend RFC 8259 compliant

[keelerm84]

The default (non-easyjson) tokenizer accepted several inputs that RFC 8259
forbids and mangled one valid input. Since the default implementation is the
source of truth for this library, align its accept/reject decisions with
encoding/json's json.Valid.

Reader (jreader/token_reader_default.go):

• Whitespace: allow only space, tab, LF, and CR between tokens instead of the
  broader unicode.IsSpace (which accepted vertical tab, form feed, NEL, NBSP).
• Numbers: reject leading zeros, a bare minus sign, and a trailing decimal
  point by walking the RFC number grammar.
• Strings: reject unescaped control characters (below 0x20).
• Strings: combine UTF-16 surrogate pairs into a single code point; lone or
  invalid surrogates decode to the replacement character, matching
  encoding/json.

Writer (jwriter/token_writer_default.go):

• Float64 returns an error for NaN and infinities rather than emitting invalid
  JSON.

Tests are default-only because the easyjson backend does not make all of the
same decisions.

---

Note

Medium Risk
Stricter parsing and different edge-case decoding may reject or change values for inputs the old tokenizer accepted; scope is core JSON I/O but behavior is intentionally aligned with encoding/json.

Overview
Aligns the default (non-easyjson) JSON tokenizer and writer with RFC 8259 and encoding/json accept/reject and decode behavior.

Reader: Token whitespace is limited to space, tab, LF, and CR (no longer all unicode.IsSpace). Number parsing follows the RFC grammar (e.g. rejects leading zeros, bare -, trailing .) and int64 overflow falls back to float parsing instead of wrapping. Strings reject unescaped controls; invalid UTF-8 and lone/malformed \u surrogates match stdlib (replacement char); valid surrogate pairs combine into one code point.

Writer: Float64 returns an error for NaN and ±Inf instead of emitting invalid JSON.

Adds default-only RFC compliance tests (easyjson backend unchanged).

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[aaron-zeisler]

Minor: I think these 5 lines are duplicate code - it looks like the exact same thing as the 2nd test case.

[aaron-zeisler]

Approved with a minor recommendation 👍 To me, this code feels finicky, but I understand its purpose, and I'm not sure how to improve it generally.

[kinyoklion]

Potential concern about non-breaking spaces, but it is JSON compliant.

[kinyoklion]

Potential concern about non-breaking spaces, but it is JSON compliant.

Looks like at least some other SDKs also reject, so we should be good.