Simplify the WriteApi request path after removing the OpenAPI client

[bednar]

Context

PR #217 removed the generated ApiClient, Configuration, and WriteService classes and introduced the internal RestClient.

The write implementation still contains generated OpenAPI request plumbing in influxdb_client_3/write_client/client/write_api.py. This makes the write path harder to maintain and leaves v2-specific generated behavior in a v3 client.

Scope

Simplify the internal WriteApi request path into three focused steps:

serialize(record)
build_write_request(...)
rest_client.request(...)

Remove or consolidate:

• _post_write_prepare;
• call_api;
• _check_operation_params;
• _sanitize_for_serialization;
• duplicated request preparation logic;
• stale generated v2 API documentation.

The refactor must preserve the current write behavior:

• v2: /api/v2/write, bucket, short precision values;
• v3: /api/v3/write_lp, db, long precision values;
• no_sync and accept_partial only for the v3 endpoint;
• gzip, timeout, retry, proxy, TLS, and error translation behavior.

Add or update tests for the v2/v3 request matrix and the existing error cases.

Acceptance criteria

• All existing unit and integration tests pass.
• Request construction is covered for both v2 and v3 endpoints.
• No references to the removed ApiClient, Configuration, or WriteService remain in the write implementation.
• The write path no longer depends on generic generated OpenAPI request helpers.
• Endpoint selection remains centralized.
• No second HTTP transport abstraction is introduced.
• No public API changes are introduced.

Out of scope

• Adding new InfluxDB endpoints.
• WritePrecision.AUTO.
• Redesigning batching or RxPY.
• Moving or renaming RestClient.
• Changes to the query transport.
• Removing or deprecating publicly reachable methods such as post_write_async; that requires a separate decision.