Unify vector setter dispatch

[mlafeldt]

Use the setter selected during vector initialization as the single correctness dispatch for chunk, row, appender, and table UDF writes.

• Adds ARRAY support to SetChunkValue.
• Writes nil values, including typed-nil *big.Int, as SQL NULL.
• Retains typed and numeric fast paths; uncommon fallback conversions may box.

BREAKING: JSON writes now match SetValue and appenders. Strings become JSON strings, []byte becomes a base64-encoded JSON string, and maps or structs become JSON objects. Use json.RawMessage for pre-serialized JSON documents.

---

Go input	Current main	After this PR
string("hello")	Writes invalid raw JSON hello; reading returns an API error wrapping json.SyntaxError	Writes "hello"; reads back as Go string "hello"
string("{\"a\":1}")	Writes raw object {"a":1}; reads back as a map	Writes "{\"a\":1}"; reads back as Go string {"a":1}
[]byte("{\"a\":1}")	Writes raw object {"a":1}; reads back as a map	Writes "eyJhIjoxfQ=="; reads back as Go string "eyJhIjoxfQ=="
map[string]any{"a": 1}	Cast error	Writes {"a":1}; reads back as a map
json.RawMessage("{\"a\":1}")	Cast error	Writes raw object {"a":1}; reads back as a map

[mlafeldt]

@taniabogatsch I think this breaking change is justified. Agree?

[taniabogatsch]

The breaking change is fine by me.

Another question, I remember the PR that added the setVectorValue interface, maybe worth checking that we don't regress here? marcboeker#276