feat: nightly hardening - bounded you.com response parsing

[mouse-value-add]

Problem

The You.com provider decoded successful JSON responses directly from without a size cap. A malformed or hostile upstream payload could force unbounded response reads and excessive memory pressure.

Approach

• Added a hard response decode limit () in .
• Switched JSON decoding to .
• Added to verify oversized payload handling is surfaced as a transient error.

Verification

• Ran: ok github.com/frugalsh/frugal/internal/provider/youcom (cached)
  ok github.com/frugalsh/frugal/internal/search (cached)
  ok github.com/frugalsh/frugal/internal/mcp/tools (cached)
• Result: all tests passed.

Risks

• Very large but valid You.com payloads above 2 MiB will now fail decode and retry/fallback instead of being processed.

Rollback plan

• Revert this PR commit to remove the size cap and restore prior decode behavior.