fix: clarify Z.AI plan presets and allow endpoint retries

[hetaoBackend]

Change

Fixes #259. Z.AI/Zhipu Coding Plan keys could be tested against the general API endpoint because onboarding pinned the ambiguous general-plan preset first. Known presets also offered no way to edit that endpoint without abandoning the draft.

• Prefer the regional Coding Plan preset in the local default order and label general presets as Z.AI API / Zhipu AI API. Explicit remote pin ordering remains authoritative.
• Show the selected URL and allow Ctrl+E editing before testing or after failure. Failed tests explicitly report that changes were not saved and retain the key/model draft for retry. No automatic endpoint or host fallback is added.
• Keep models.dev IDs and URLs intact. They are selection metadata, not bundled inference-provider IDs: saved custom_provider connections resolve their explicit URL. The bundled snapshot already contains GLM-5.3; no third-party catalog or fix: route Zhipu coding-plan base URLs to the Zhipu token counter #260 token-counter changes are needed.
• Document the actual entry point: /model → + Add 3rd-party provider….

Validation

• Focused catalog, onboarding and candidate-management suites: 144 passed.
• pnpm verify, full profile on clean committed HEAD 2154c04c805832369596bcb052ece6aa86aea973, macOS / Node v26.4.0: PASS.
• Added offline coverage for both regions, plan labels/URLs, failure without persistence or fallback, explicit editing/retry, and saved-provider resolution. Built CLI BYOK coverage reads back config.yaml and verifies actual inference preserves /api/coding/paas/v4/chat/completions against a local server.
• Isolated built-CLI PTY check: real picker order/labels, GLM-5.3 search, URL editing, masked synthetic key, failed-test message, retained draft, and no provider saved on failure. External networking was blocked.
• Performance: basic; no history, token counting, storage implementation, streaming or execution-loop changes.
• NOT RUN: real Z.AI/Zhipu keys or services; Linux/Windows local acceptance. Remote CI is reported separately.

Publication and contribution checks

• Changes are contributed under the existing applicable licenses; existing notices are preserved.
• No credentials, account data, real user content, internal history or private review artifacts are included.
• Only existing files changed; existing Vitest suite declarations apply and source inventory regeneration is unnecessary.
• English usage documentation is updated; offline results are not described as live-service acceptance.

Maintainer handoff

Publication scope/license changes: none. Shared-source port: pending.

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