feat(security): add configurable security bypass for HTTP mode (#44)

Add ability to disable OAuth2 security in HTTP mode for local development
and testing scenarios. Security can be toggled via spring.security.enabled
property:

- spring.security.enabled=true (default): Full OAuth2 authentication
- spring.security.enabled=false: All requests permitted, @PreAuthorize bypassed

Comprehensive guide for setting up Keycloak as OAuth2/OIDC identity
provider for Solr MCP Server in HTTP mode.

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Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: adityamparikh