feat(providers): split _coding api_key_env from main provider (#82)

Each `_coding` provider entry previously declared the same `api_key_env`
as its standard counterpart, causing one credential to silently activate
two providers. Users who only signed up for the per-token API endpoint
saw the Coding Plan endpoint's models mixed into their model picker
without intent.

Rename so each pair uses an independent env var:

  byteplus-coding    BYTEPLUS_API_KEY   -> BYTEPLUS_CODING_API_KEY
  volcengine-coding  VOLCENGINE_API_KEY -> VOLCENGINE_CODING_API_KEY
  zai-coding         ZHIPU_API_KEY      -> ZAI_CODING_API_KEY
  zhipu-coding       ZHIPU_API_KEY      -> ZHIPU_CODING_API_KEY

This is a breaking change: existing users must set the new env vars.
The librefang daemon will be updated separately to recognize the old
env vars as a deprecated fallback during a migration window. See
librefang/librefang#3278 for the design discussion and rollout plan.

Out of scope (different problem class, tracked in the same issue):

  - zai/zhipu cross-region key sharing (both still share ZHIPU_API_KEY)
  - github-copilot/microsoft cross-product GITHUB_TOKEN reuse

Refs librefang/librefang#3278

providers/volcengine-coding.toml

@@ -4,6 +4,6 @@
 [provider]
 id = "volcengine_coding"
 display_name = "Volcano Engine Coding Plan"
-api_key_env = "VOLCENGINE_API_KEY"
+api_key_env = "VOLCENGINE_CODING_API_KEY"
 base_url = "https://ark.cn-beijing.volces.com/api/coding/v3"
 key_required = true