MCP servers may accept `initialize` unauthenticated but return `401`
with a `WWW-Authenticate` challenge only on a later request such as
`tools/list` or `tools/call`. Previously Zed only started the Auth flow
when `initialize` itself was challenged, so these servers failed
opaquely and stayed stuck in `Running` with a dead client.

`ContextServerStore` now handles `TransportError::AuthRequired` returned
from any request, not just startup: it runs Auth discovery and
transitions the server into `AuthRequired` so the UI offers to
authenticate. The discovery logic is shared with the startup path, and
the tool/prompt request sites route their errors through it.

Release Notes:

- Fixed MCP servers that require auth only on tool calls (not on
`initialize`) failing to prompt for authentication.

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Co-authored-by: Tom Houlé <tom@tomhoule.com>