Refactor write HTTP stack: introduce internal `_RestClient` and bypass OpenAPI `WriteService`

[bednar]

Background

influxdb3-python still uses a legacy OpenAPI-style HTTP stack for write requests:

• WriteApi -> WriteService -> ApiClient -> RESTClientObject -> Configuration

This stack was inherited from the InfluxDB v2 client and is now heavier than needed for the InfluxDB 3 client. Sign-in/sign-out services have already been removed, but write requests still depend on the generated-style ApiClient and WriteService layers.

Goal

Introduce a small internal _RestClient for HTTP-based endpoints and route writes through it directly, while preserving the current public write API behavior.

Scope

1. Add an internal _RestClient

Suggested location:

influxdb_client_3/write_client/_sync/rest_client.py

Responsibilities:

• Build request URLs from base host, endpoint path, and query params.
• Add default headers:
  • Authorization
  • User-Agent
• Send HTTP requests using urllib3.
• Support existing write HTTP options:
  • verify_ssl
  • ssl_ca_cert
  • cert_file
  • cert_key_file
  • cert_key_password
  • ssl_context
  • proxy
  • proxy_headers
  • connection_pool_maxsize
  • request timeout in milliseconds
  • gzip via enable_gzip and gzip_threshold
• Raise the existing ApiException or compatible error type for non-2xx responses.

2. Route write requests through _RestClient

Refactor WriteApi._post_write(...) so it no longer calls WriteService.post_write(...).

Preserve current endpoint behavior:

• use_v2_api=True
  • endpoint: /api/v2/write
  • query params: org, bucket, precision
• use_v2_api=False
  • endpoint: /api/v3/write_lp
  • query params: org, db, precision
  • optional: no_sync=true
  • optional: accept_partial=false

Preserve current validation:

• use_v2_api=True with no_sync=True raises:
  • ValueError("invalid write options: no_sync cannot be used with use_v2_api")

Preserve current error translation:

• 405 on /api/v2/write suggests use_v2_api=False
• 405 on /api/v3/write_lp suggests use_v2_api=True
• partial write responses are translated to InfluxDBPartialWriteError

Acceptance criteria

• Write requests work through both /api/v2/write and /api/v3/write_lp.
• accept_partial, no_sync, and use_v2_api keep their current behavior.
• gzip, timeout, proxy, and TLS options still work.
• Runtime write path no longer depends on WriteService, ApiClient, or RESTClientObject.
• Existing high-level public APIs remain compatible:
  • InfluxDBClient3
  • WriteOptions
  • client.write(...)
  • client.flush()
  • client.close()
• Tests that currently cover WriteService/ApiClient are replaced or updated to cover _RestClient and the new write path directly.