[CHORE][REFACTOR]: Retire legacy mcpgateway.validators references

[crivetimihai]

Overview

Clean up legacy mcpgateway.validators compatibility references so docs/tests point to the canonical mcpgateway.common.validators, and decide whether to retire the shim.

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Current State

• mcpgateway/validators.py re-exports SecurityValidator for backward compatibility.
• docs/docs/architecture/security-features.md references mcpgateway/validators.py (non-canonical path).
• tests/security/test_input_validation.py includes a coverage comment with --cov=mcpgateway.validators.
• No in-repo imports of mcpgateway.validators found via search.

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Proposed Implementation

1. Update docs mention to mcpgateway.common.validators.
2. Update/remove the --cov=mcpgateway.validators comment in tests/security/test_input_validation.py.
3. Confirm external consumers and then either:
   • remove mcpgateway/validators.py, or
   • keep it with a deprecation note + removal timeline.

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Acceptance Criteria

• No references to mcpgateway/validators.py or mcpgateway.validators remain in the repo.
• SecurityValidator usage continues via mcpgateway.common.validators.
• Decision recorded on shim removal vs. deprecation.