fix(models): route falsey mapped providers

[hsusul]

Summary

This pull request fixes MultiProvider routing for explicitly mapped provider objects that evaluate to false. The prefix resolver now distinguishes a missing map entry with is not None, so an explicit MultiProviderMap entry continues to win over built-in and unknown-prefix behavior regardless of the provider's truth value.

Test plan

• Added test_provider_map_routes_to_falsey_provider, which failed with UserError: Unknown prefix: custom before the fix and passes afterward.
• tests/models/test_map.py: 15 passed.
• Ran .agents/skills/code-change-verification/scripts/run.sh:
  • format, lint, mypy, and pyright passed.
  • the managed environment's full suite reached 5,747 passed and 3 skipped; 22 local websocket and Unix sandbox tests failed because the execution sandbox denied local listener/process operations with Operation not permitted.
  • an unsandboxed rerun could not start because the environment's approval service had exhausted its usage allowance. The remaining full-suite gate is therefore left to CI.

Issue number

N/A - found through code-path review; no existing issue or pull request covers falsey mapped-provider routing.

Checks

• I've added new tests, if relevant
• I've run .agents/skills/code-change-verification/scripts/run.sh
• I've confirmed all verification steps pass
• If using Codex, I've run /review before submitting this PR