fix(config): reject non-finite provider costs

[mouse-value-add]

Summary

• reject non-finite values (, , ) during config validation
• keep existing non-negative validation intact
• add table-driven tests covering all non-finite YAML float spellings

Why

feeds provider routing and comparisons. Non-finite values can cause undefined ordering and correctness issues in cost-based selection.

Validation

• ? github.com/frugalsh/frugal/cmd/frugal [no test files]
  ok github.com/frugalsh/frugal/internal/browse (cached)
  ok github.com/frugalsh/frugal/internal/config (cached)
  ok github.com/frugalsh/frugal/internal/extract (cached)
  ok github.com/frugalsh/frugal/internal/install (cached)
  ok github.com/frugalsh/frugal/internal/mcp (cached)
  ok github.com/frugalsh/frugal/internal/mcp/tools (cached)
  ok github.com/frugalsh/frugal/internal/obs (cached)
  ok github.com/frugalsh/frugal/internal/provider/browserless (cached)
  ok github.com/frugalsh/frugal/internal/provider/firecrawl (cached)
  ok github.com/frugalsh/frugal/internal/provider/goreadability (cached)
  ok github.com/frugalsh/frugal/internal/provider/marginalia (cached)
  ok github.com/frugalsh/frugal/internal/provider/searxng (cached)
  ok github.com/frugalsh/frugal/internal/provider/serper (cached)
  ok github.com/frugalsh/frugal/internal/provider/youcom (cached)
  ok github.com/frugalsh/frugal/internal/routing (cached)
  ok github.com/frugalsh/frugal/internal/search (cached)

Backward compatibility

• no runtime behavior changes for valid configs
• invalid configs with non-finite costs now fail fast at startup with a clear error