fix(server)!: remove inline config from /v1/checks endpoint

[m-misiura]

Description

The inline config specification should be dropped from v1/checks endpoint as it mixes control and data planes. This impacts PR#2205, so it's probably prudent to review this first and then I will subsequently revise the aforementioned PR

Related Issue(s)

No issue was filed, but this was discussed previously

Verification

Revised tests pass:

 uv run --locked pytest tests/server/test_guardrail_checks.py -v

tests/server/test_guardrail_checks.py::test_status_mapping[passed-passed] PASSED [  6%]
tests/server/test_guardrail_checks.py::test_status_mapping[modified-modified] PASSED [ 13%]
tests/server/test_guardrail_checks.py::test_status_mapping[blocked-blocked] PASSED [ 20%]
tests/server/test_guardrail_checks.py::test_content_returned_on_passed PASSED [ 26%]
tests/server/test_guardrail_checks.py::test_content_returned_on_modified PASSED [ 33%]
tests/server/test_guardrail_checks.py::test_content_returned_on_blocked PASSED [ 40%]
tests/server/test_guardrail_checks.py::test_rail_null_on_passed PASSED   [ 46%]
tests/server/test_guardrail_checks.py::test_config_id_resolves PASSED    [ 53%]
tests/server/test_guardrail_checks.py::test_default_config_used_when_none_specified PASSED [ 60%]
tests/server/test_guardrail_checks.py::test_empty_messages_returns_422 PASSED [ 66%]
tests/server/test_guardrail_checks.py::test_no_config_no_default_returns_422 PASSED [ 73%]
tests/server/test_guardrail_checks.py::test_colang_v2_returns_422 PASSED [ 80%]
tests/server/test_guardrail_checks.py::test_get_rails_failure_returns_422 PASSED [ 86%]
tests/server/test_guardrail_checks.py::test_check_async_failure_returns_500 PASSED [ 93%]
tests/server/test_guardrail_checks.py::test_context_prepended_to_messages PASSED [100%]

============================== 15 passed in 2.12s ==============================

AI Assistance

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• AI tools were used; a human reviewed and can explain every change (tool: Claude Code).

Checklist

• I've read the CONTRIBUTING guidelines.
• This PR links to a triaged issue assigned to me.
• My PR title follows the project commit convention.
• I've updated the documentation if applicable.
• I've added tests if applicable.
• I've noted any verification beyond CI and any checks I couldn't run.
• I did not update generated changelog files manually.
• I addressed all CodeRabbit, Greptile, and other review comments, or replied with why no change is needed.
• @mentions of the person or team responsible for reviewing proposed changes.

[Pouyanpi]

Thank you @m-misiura for fixing this 👍🏻