ENG-847: Fix scratchpad web_search() on the minds-cloud gateway

[ea-rus]

Problem

Scratchpad web_search() returns an empty string under the minds-cloud provider (the
standard config), so the agent silently loses web access and falls back to scraping.
Affects real cowork/anton usage, not just evals.

Cause: the scratchpad picks its web "flavor" by provider name, but OpenAIProvider.name
is always "openai" even on the minds gateway — so it chose the OpenAI Responses API and
dispatched web_search to an endpoint the minds gateway doesn't implement (minds serves it
over the chat.completions passthrough) → empty. The gateway itself is fine (a direct
chat.completions call with tools=[{"type":"web_search"}] returns real results).

Fix

Resolve the flavor by HOST, not provider name: minds/mdb.ai base URL → passthrough.
Extracted as OpenAIProvider.resolve_web_flavor(), called from scratchpad_boot.py.
All prior cases unchanged; only the broken openai-name + minds-base case now works.

Repro

cowork-server on minds_cloud, then in the scratchpad:
print(repr(web_search("current US CPI year-over-year rate")))
Before: ''. After: a non-empty answer with sources.

Tests

tests/test_scratchpad_flavor.py (new, 7 cases); test_scratchpad.py + test_web_tools.py
still green (107 passed).

Fixes https://linear.app/mindsdb/issue/ENG-847/scratchpad-web-search-silently-returns-empty-on-the-minds-cloud