[#4409] JSON-encode scalar projections when streaming to a response body

[jeremydmiller]

Summary

Closes #4409.

WriteArray / StreamJsonArray / WriteOne / StreamJsonOne against a scalar projection — .Select(x => x.SomeEnum) under EnumStorage.AsString, or any .Select(x => x.StringProperty) — used to emit raw Postgres bytes between the array brackets and commas. data->>'X' comes back as text with no JSON quoting, so the body landed as

[FooValue,BarValue]

— invalid JSON, with downstream System.Text.Json blowing up:

'F' is an invalid start of a value. Path: $[0] | LineNumber: 0 | BytePositionInLine: 1.

Numeric / boolean projections happened to look like valid JSON literals ([1,2,3]), which is why the bug only surfaced for string-valued scalars.

Fix

Marten.Services.JsonStreamingExtensions gets a new WriteJsonValueAsync(NpgsqlDataReader, int ordinal, Stream, CancellationToken) helper that branches on reader.GetDataTypeName(ordinal):

• jsonb / json → copy the field stream byte-for-byte with the existing SOH-skip (unchanged document-streaming behavior).
• DBNull → write the JSON null literal.
• Everything else → materialize the .NET value via reader.GetFieldValueAsync<object> and round-trip through JsonSerializer.SerializeAsync with the column's CLR type. Strings get JSON-quoted and escaped (handles embedded ", \, control characters); numerics, booleans, dates, and enum-as-string come out as their JSON literal / quoted-string representation.

Routes the existing StreamMany / StreamOne reader extensions and the inline copy in OneResultHandler.StreamJson through the new helper. Document-streaming paths (whole-doc jsonb column) keep the previous fast-path verbatim — WriteJsonValueAsync's first branch is the same SOH-skip copy that was there before.

Test plan

Five new regression tests in Bug_4409_streaming_scalar_string_projection:

• stream_array_of_enum_as_string_projection_emits_valid_json — primary repro from the issue (enum-as-string).
• stream_array_of_string_projection_emits_valid_json — Select(x => x.Name) case the issue also calls out.
• stream_array_of_string_projection_escapes_embedded_special_characters — pins the JSON-escaping path against quotes, backslashes, and newlines in the projected text.
• stream_array_of_int_projection_still_emits_valid_json — guards against regressing the numeric path that already produced valid JSON.
• stream_array_of_jsonb_documents_still_emits_valid_json — guards the whole-document jsonb path (must take the fast-path branch).

Pre-fix: 4/5 fail with JsonReaderException on body parse. Post-fix: 5/5 green.

Broader runs:

• Build clean (0 errors)
• DocumentDbTests — 987 pass, 1 pre-existing skip, 0 fail (net10.0)
• LinqTests — 1257 pass, 1 pre-existing skip, 0 fail (net10.0)
• CI matrix

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