feat: Add LOCALAI_DISABLE_MCP environment variable to disable MCP support (#8816)

* feat: Add LOCALAI_DISABLE_MCP environment variable to disable MCP support

- Added DisableMCP field to RunCMD struct in core/cli/run.go
- Added LOCALAI_DISABLE_MCP environment variable support
- Added DisableMCP field to ApplicationConfig struct
- Added DisableMCP AppOption function
- Updated MCP endpoint routing to check appConfig.DisableMCP
- When LOCALAI_DISABLE_MCP is set to true/1/yes, MCP endpoints are not registered

When set, all MCP functionality is disabled and appropriate error messages
are returned to users.

Use Cases:
- Security-conscious deployments where MCP is not needed
- Reducing attack surface
- Compliance requirements that prohibit certain protocol support

Environment variable: LOCALAI_DISABLE_MCP=true

Signed-off-by: localai-bot <localai-bot@users.noreply.github.com>

* docs: Add documentation for LOCALAI_DISABLE_MCP environment variable

- Add section explaining how to disable MCP support using environment variable
- Document use cases for disabling MCP
- Provide examples for CLI and Docker usage

Signed-off-by: localai-bot <localai-bot@users.noreply.github.com>

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Signed-off-by: localai-bot <localai-bot@users.noreply.github.com>
Co-authored-by: localai-bot <localai-bot@users.noreply.github.com>