Reject unpaired surrogates in JsonWriter STRICT mode

[sushant-me]

JsonReader rejects unpaired UTF-16 surrogate characters in strict mode (that was #3113 / #3116), but JsonWriter has no equivalent validation, so the two sides of the API disagree about what a strict JSON string is.

An unpaired surrogate in a Java String has no faithful JSON encoding: a JSON string is a sequence of Unicode characters (RFC 8259 section 7). Two consequences today, both silent:

• JsonWriter with Strictness.STRICT writes the value anyway, producing a document that its own Javadoc promises conforms to RFC 8259 but which JsonReader in the same strictness mode then refuses to read back. Gson's own default output is not readable by Gson.
• On the documented write-to-UTF-8 path the character becomes ?, so data is silently lost.

What this changes

JsonWriter.string(...) is the single sink for both names and values, so validating there covers value(String), name(String) and deferred names alike:

private void string(String value) throws IOException {
  validateString(value);
  ...

validateString mirrors JsonReader.validateString exactly, including the message, so a caller that catches one path understands the other.

Scope

The check is limited to Strictness.STRICT. LEGACY_STRICT and LENIENT are unchanged, so no existing caller sees different output. That mirrors how the read side was introduced, and is why this is not a behaviour change for anyone who did not already opt into strict mode.

Tests

Adds the writer-side coverage that never existed:

• testStrictModeRejectsUnpairedSurrogates — lone high, lone low, embedded, and reversed pair.
• testStrictModeRejectsUnpairedSurrogateInName — the name path (rejection surfaces when the entry is completed, because names are written lazily).
• testStrictModeAllowsPairedSurrogates — a valid pair is written and then read back in strict mode, which is the round-trip property the bug broke.
• testNonStrictModesStillAllowUnpairedSurrogates — pins the legacy behaviour so it cannot regress accidentally.

mvn -pl gson test passes: 4671 tests, 0 failures, 0 errors.

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[sushant-me]

@googlebot I signed it!