feature: Propagate error information from nemo check

[evaline-ju]

Feature Description

In #17 a plugin was added to interface with the "nemo check server" (server that provides /v1/guardrails/checks). When the nemo check rails indicate a request/response should be blocked (status = blocked instead of status = success), information on what checks are run will be included in rails_status and included as result.metadata. We want to make sure this information can be provided in the final MCP error provided by the adapter and that any other useful "violation" details from the plugin are surfaced in the MCP error.

Proposed Solution

• Put rails_status information on MCP error.data
• Make sure MCP error code is reflective of the violation
• Make sure error.message reflects the violation.reason and violation.description

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Additional Context

No response

[evaline-ju]

Incorporated some consistency across pre_tool_invoke and post_tool_invoke through #32 but amount of info should still be discussed

[evaline-ju]

Decision: don't surface all the rails_status json (just keep to "Check tool rails:blocked...")