fix: make auto-continuation settings-only

[Brian Krabach (bkrabach)]

What changed

• Remove the “Continue truncated responses” field from provider setup metadata. auto_continue remains an optional settings-only override; omitting it keeps continuation enabled, and explicit false (including legacy string values) remains honored.
• Keep per-call overrides, partial-response/usage handling, API request behavior, and the advertised capability unchanged.
• Update the README with the settings-only opt-out example.

This is the vLLM part of the settings-only continuation release; the CLI override-preservation fix is in amplifier-app-cli#366. It does not change input truncation, timeouts, or impose a completion deadline. No breaking changes intended.

Why

The provider setup wizard should not ask about this optional advanced setting. Removing its metadata prompt keeps setup focused while preserving current defaults and explicit settings. The CLI preservation fix ensures reconfiguration does not discard the override.

How to verify

• uv run pytest -q -m "not live": 369 passed, 1 live test deselected.
• Focused continuation suite: 55 passed.
• Installed-package acceptance across CLI, OpenAI, and vLLM: 50 cases passed; mocked transport, no paid inference.
• README and provider behavior checked against the qualified candidate file hashes.