fix(grok): stop rejecting hosted web search domain and location filters

[AlexanderKugel]

Fixes #123.

Problem

Claude Code can declare Anthropic hosted web search with allowed_domains, blocked_domains, or user_location. Grok's hosted web_search has no equivalent fields, so the proxy returns HTTP 400 without calling Grok.

Claude Code surfaces that 400 to the model as an error tool_result. The model then retries the same search without the filter, and the retry is accepted and runs unrestricted. The rejection therefore costs an extra round and the caller's constraint is lost anyway.

Change

CCP_SEARCH_CONSTRAINTS selects the behaviour for providers that cannot enforce these fields:

Value	Behaviour
soft (default)	Drop the fields and turn each one into an instruction appended to the upstream request's instructions field.
warning	Drop the fields, log a warning, send the search unrestricted.
hard	Keep the current HTTP 400.

hard preserves today's behaviour for anyone who wants it. The 400 message now names every unsupported field in the request instead of only the first one found.

max_uses stays dropped, as before.

Example of the appended instruction:

You are only allowed to search allowed_domains=["langchain.com","docs.langchain.com","blog.langchain.com"].

Values are copied verbatim from the caller. The proxy does not parse them.

Why soft is the default

Measured with 10 live searches per group, same query, three allowed domains:

Group	Runs	Total [in-domain, off-domain]	In-domain share
soft	10/10	[161, 0]	100.0%
no domain list	10/10	[147, 210]	41.2%

Grok applies the instruction by adding site: filters to its own search queries. soft is a bias, not enforcement; the model can ignore it.

Notes

• Unit tests cover all three modes, the multi-field error message, and the verbatim copying of user_location.
• No version bump and no changelog entry, as those are yours to make.
• cargo fmt --check and cargo clippy --lib -- -D warnings are clean. The library suite passes except providers::codex::tests::cancellation_while_replacement_startup_is_blocked_aborts_request_state, which fails on main without this change and is reported in Test cancellation_while_replacement_startup_is_blocked_aborts_request_state fails: replacement startup never reaches the blocked handshake #124.