refactor: route HTTP gateway JSON through engine type inference

[mosuka]

Summary

• Route laurus-server HTTP gateway JSON ingestion through laurus::engine::type_inference::infer_from_json so REST inputs follow the same inference rules as schema-less ingestion.
• Drop the hand-rolled converter (numeric-array → vector, canonical-only geo keys, silent text fallbacks) in favor of the engine helper, which now reaches multi-valued numerics following Add native support for multi-valued numeric fields (Int64Array / Float64Array) #281.
• Surface inference errors as HTTP 400 with the offending field name instead of quietly text-stringifying the value.

Behavior changes (pre-release, allowed)

Input	Before	After
[1, 2, 3]	VectorValue (legacy)	Int64ArrayValue (multi-valued integer)
[1.0, 2.5]	VectorValue	Float64ArrayValue (multi-valued float)
[]	TextValue("[]")	NullValue (skipped)
[1, "x"]	TextValue("[1,\"x\"]")	HTTP 400
{"lat":..., "lon":...}	TextValue	GeoValue
{"latitude": 200, "longitude": 0}	accepted	HTTP 400
{"foo":"bar"}	TextValue("{...}")	HTTP 400

Vector REST ingestion is preserved: declared vector fields keep accepting numeric arrays because coerce_to_vector already handles Int64Array/Float64Array (PR #286).

Implementation

• laurus-server/src/gateway/convert.rs — json_value_to_proto is now Result<v1::Value, String>, delegating to infer_from_json + data_value_to_proto. json_to_proto_document propagates per-field errors with the field name.
• laurus-server/src/convert/document.rs — data_value_to_proto / data_value_from_proto made pub so the gateway can lower the inferred DataValue.
• laurus/src/lib.rs — re-export InferredValue / infer_from_json / infer_option_from_data_value for downstream crates.
• laurus/src/engine/type_inference.rs — drop #![allow(dead_code)]; refresh the doc table to include array → multi-valued numeric mappings (the "not yet supported" note is now stale).
• Docs: new "JSON Field Value Inference" section in docs/src/laurus-server/http_gateway.md (+ JA translation).

Closes #282

Test plan

• cargo fmt --check
• cargo clippy --all-targets --workspace --exclude laurus-ruby -- -D warnings (laurus-ruby skipped, ruby.h env issue, same as PR feat: multi-valued numeric DataValue and schema flag (PR1/2 of #281) #286)
• cargo test -p laurus-server --lib — 23/23 pass (14 new gateway cases: array roundtrips, geo aliases, range checks, mixed-array error, non-geo object error, document-level field-name error)
• cargo test -p laurus — 698 unit + integration tests pass
• markdownlint-cli2 "docs/src/**/*.md" "docs/ja/src/**/*.md" — 0 errors
• mdbook build docs && mdbook build docs/ja