chore: default authentication path to /* in platform-http-main (4.18.x)

[Croway]

Backport of #22474 to camel-4.18.x.

Summary

• Default the authentication path to /* when not explicitly configured in BasicAuthenticationConfigurer and JWTAuthenticationConfigurer
• Extract shared path resolution logic into MainAuthenticationConfigurer default method
• Add tests for non-root context path and selective path authentication
• Add upgrade guide entry for 4.18.1 → 4.18.2

Test plan

• Existing auth tests pass (Basic + JWT)
• New test: non-root context path with implicit auth path
• New test: explicit authenticationPath for selective protection