Fix and speed up Windows Backend CI

[benclawbot]

Summary

• restore Windows-only anyhow::Context import for the Word tool so Windows checks compile
• add explicit optional intent schema fields for first-party tools caught by the schema test
• remove unused Windows computer-tool imports
• reduce long CI timeouts, gate coverage behind fast Windows checks, and narrow coverage to the schema smoke test so failures stop quickly

Verification

• cargo test -p iagent --lib tool::tests::first_party_tool_definitions_include_optional_intent_explicitly -- --exact --nocapture
• cargo check -p iagent --lib --message-format=short

Local Windows-target verification is blocked on this machine by missing MSVC linker/toolchain (link.exe missing natively; WSL uses GNU cc for an MSVC target).

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