chore(docs): add and refine package-level comments across modules

- Added detailed package-level comments to improve documentation coverage.
- Clarified parameter descriptions, return types, and functionality of exported methods across packages.
- Enhanced overall code readability and API documentation consistency.

internal/misc/header_utils.go

@@ -1,3 +1,6 @@
+// Package misc provides miscellaneous utility functions for the CLI Proxy API server.
+// It includes helper functions for HTTP header manipulation and other common operations
+// that don't fit into more specific packages.
 package misc
 
 import (
@@ -5,6 +8,16 @@ import (
 	"strings"
 )
 
+// EnsureHeader ensures that a header exists in the target header map by checking
+// multiple sources in order of priority: source headers, existing target headers,
+// and finally the default value. It only sets the header if it's not already present
+// and the value is not empty after trimming whitespace.
+//
+// Parameters:
+//   - target: The target header map to modify
+//   - source: The source header map to check first (can be nil)
+//   - key: The header key to ensure
+//   - defaultValue: The default value to use if no other source provides a value
 func EnsureHeader(target http.Header, source http.Header, key, defaultValue string) {
 	if target == nil {
 		return