Add a first-class Z AI Coding Plan provider (subscription endpoint api.z.ai/api/coding/paas/v4)

[ruttybob]

Problem

The stock zai provider (coworker/providers/registry.py, base_url https://api.z.ai/api/paas/v4, env ZAI_API_KEY) covers only pay-as-you-go API access. Z AI's Coding Plan is a separate subscription product with its own endpoint (https://api.z.ai/api/coding/paas/v4) and its own key. A plan subscriber has no first-class slot: today they would have to hand-edit the zai endpoint or abuse the "point OpenAI at another gateway" trick.

Proposal

Add a first-class registry entry following the existing plan-provider precedent (ark-agent-plan / ark-agent-plan-cn):

• id: zai-coding, title: Z AI Coding Plan
• base_url prefilled to https://api.z.ai/api/coding/paas/v4
• env_key: ZAI_CODING_API_KEY
• recommended model from the Coding Plan lineup (GLM coding models)
• endpoint_help noting it requires a Coding Plan subscription and is separate from stock Z AI

Notes

• Stock zai stays untouched; builtin-name collision rejection already guards routing ambiguity.
• Composes with thinking controls like any other compat provider.

[ruttybob]

Related: #547 — a generic user-defined compat-provider mechanism would make entries like this one unnecessary as one-off registry PRs.

[pranjalparmar]

I've opened a PR for this at #583 which implements the Z AI Coding Plan provider along with dynamic model selection in the settings GUI.