fix: ISO8601 should take priority over yyyyMMddHHmmss in Instant write, for issue #4003

[wenshao]

Summary

• When the JSONWriter.Context has dateFormat="iso8601" (the runtime form that fastjson1's SerializerFeature.UseISO8601DateFormat resolves into) ISO8601 must take priority over any string-pattern shape flags such as yyyyMMddhhmmss19/14/8/10.
• Previously the codec evaluated those flags purely against the codec instance + context's pattern flags, so a setup that had been initialized with dateFormat="yyyy-MM-dd HH:mm:ss" and later switched to ISO8601 could still hit the yyyyMMddhhmmss19 path and produce "2026-02-24 14:50:00" instead of "2026-02-24T14:50:00Z".
• Compute a single iso8601 effective flag and gate the yyyyMMddhhmmss shape paths on !iso8601, so an explicit ISO8601 request always wins.

Fixes #4003.

Test plan

• New Issue4003 test covering: ISO8601 alone, ISO8601 after a default yyyy-MM-dd HH:mm:ss, default yyyy-MM-dd HH:mm:ss without ISO8601 (regression), and no-format default.
• All *Instant*, *ISO8601*, *DateTimeCodec*, Issue* tests pass (37 tests).

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