v2: Implement HTTP transport layer — replace swagger-generated code

[oss-amikos]

Context

Currently the client relies on Swagger/OpenAPI codegen for the HTTP layer. We want to own the HTTP transport entirely — fewer dependencies, full control over serialization, better error handling.

Related: #43 (Move away from swagger generated APIs)

Requirements

Internal HttpClient

A package-private HTTP client that the Client and Collection implementations use. NOT part of the public API.

// Package-private
class HttpTransport implements AutoCloseable {
    // Core HTTP methods
    <T> T get(String path, Class<T> responseType);
    <T> T get(String path, TypeToken<T> responseType);  // for generics
    <T> T post(String path, Object body, Class<T> responseType);
    <T> T post(String path, Object body, TypeToken<T> responseType);
    void delete(String path);
    <T> T put(String path, Object body, Class<T> responseType);

    void close();
}

Dependencies

• OkHttp — HTTP client (keep, it's solid)
• Gson — JSON serialization (keep, already in use)
• Remove — swagger-codegen-maven-plugin, swagger annotations, all generated code

Serialization

• Use Gson TypeToken for generic types (e.g., List<CollectionModel>) — this fixes the generic deserialization bugs in PR [TST] Add v2 API #80
• Register custom Gson type adapters for:
  • Where → JSON map
  • WhereDocument → JSON map
  • Include enum ↔ string
  • DistanceFunction enum ↔ string
• All request/response objects use Gson @SerializedName where field names differ from Java convention

Error Handling

• Parse error responses from Chroma (JSON with error and message fields)
• Map HTTP status codes to exception hierarchy (v2: Design exception hierarchy and error handling strategy #86)
• Wrap IOException as ChromaConnectionException

URL Building

• Proper URL construction — no string concatenation for query parameters
• URL-encode path segments and query values
• Base path: /api/v2

Acceptance Criteria

• HttpTransport class with get/post/delete/put methods
• Generic deserialization via TypeToken
• Custom Gson type adapters for Where, WhereDocument, Include, DistanceFunction
• Error response parsing and exception mapping
• Proper URL building with encoding
• Configurable timeouts via OkHttp
• Auth header injection via AuthProvider
• Default headers support
• Remove swagger-codegen from pom.xml
• Remove all generated source code dependencies

Depends On

• v2: Design auth provider API — Token, Basic, X-Chroma-Token, custom headers #85 (AuthProvider)
• v2: Design exception hierarchy and error handling strategy #86 (Exception hierarchy)

Supersedes

• Move away from swagger generated APIs #43 (Move away from swagger generated APIs)