Add dataplane config lookup diagnostics

[lucarlig]

Summary

• add captured-format dataplane logs around Redis user-config lookup failures
• log request method/path/subject while the subject is still the dataplane config key
• replace full UserConfig validator dumps with captured-format presence/count messages
• log missing virtual-host config and bad virtual-host path extraction with stable method-prefixed messages

Impact

• no behavior change
• no JWTs, authorization headers, token secrets, Redis key/value bytes, full user configs, or backend credentials are logged
• JWT sub is logged temporarily because it is currently the Redis config identifier; follow-up control-plane issue: [CF-DATAPLANE] Use opaque subject identifiers for dataplane config keys IBM/mcp-context-forge#5462
• gives enough signal to distinguish Redis miss, Redis error, decode failure, and missing virtual host during live integration runs

Validation

• cargo fmt
• cargo test -p contextforge-gateway-rs-lib
• cargo clippy -p contextforge-gateway-rs-lib --all-targets --all-features
• git diff --check