feat(serialization): implement Gin JSON/YAML binding

- Introduce SubstituteEnvReader that replaces ${VAR} patterns with environment variable
  values, properly quoted for JSON/YAML compatibility
- Gin bindings (JSON/YAML) that use the environment-substituting reader
  for request body binding with validation support

internal/serialization/reader.go

@@ -0,0 +1,146 @@
+package serialization
+
+import (
+	"bytes"
+	"io"
+)
+
+type SubstituteEnvReader struct {
+	reader io.Reader
+	buf    []byte // buffered data with substitutions applied
+	err    error  // sticky error
+}
+
+func NewSubstituteEnvReader(reader io.Reader) *SubstituteEnvReader {
+	return &SubstituteEnvReader{reader: reader}
+}
+
+const peekSize = 4096
+const maxVarNameLength = 256
+
+func (r *SubstituteEnvReader) Read(p []byte) (n int, err error) {
+	// Return buffered data first
+	if len(r.buf) > 0 {
+		n = copy(p, r.buf)
+		r.buf = r.buf[n:]
+		return n, nil
+	}
+
+	// Return sticky error if we have one
+	if r.err != nil {
+		return 0, r.err
+	}
+
+	var buf [2 * peekSize]byte
+
+	// Read a chunk from the underlying reader
+	chunk, more := buf[:peekSize], buf[peekSize:]
+	nRead, readErr := r.reader.Read(chunk)
+	if nRead == 0 {
+		if readErr != nil {
+			return 0, readErr
+		}
+		return 0, io.EOF
+	}
+	chunk = chunk[:nRead]
+
+	// Check if there's a potential incomplete pattern at the end
+	// Pattern: ${VAR_NAME}
+	// We need to check if chunk ends with a partial pattern like "$", "${", "${VAR", etc.
+	incompleteStart := findIncompletePatternStart(chunk)
+
+	if incompleteStart >= 0 && readErr == nil {
+		// There might be an incomplete pattern, read more to complete it
+		incomplete := chunk[incompleteStart:]
+		chunk = chunk[:incompleteStart]
+
+		// Keep reading until we complete the pattern or hit EOF/error
+		for {
+			// Limit how much we buffer to prevent memory exhaustion
+			if len(incomplete) > maxVarNameLength+3 { // ${} + var name
+				// Pattern too long to be valid, give up and process as-is
+				chunk = append(chunk, incomplete...)
+				break
+			}
+			nMore, moreErr := r.reader.Read(more)
+			if nMore > 0 {
+				incomplete = append(incomplete, more[:nMore]...)
+				// Check if pattern is now complete
+				if idx := bytes.IndexByte(incomplete, '}'); idx >= 0 {
+					// Pattern complete, append the rest back to chunk
+					chunk = append(chunk, incomplete...)
+					break
+				}
+			}
+			if moreErr != nil {
+				// No more data, append whatever we have
+				chunk = append(chunk, incomplete...)
+				readErr = moreErr
+				break
+			}
+		}
+	}
+
+	substituted, subErr := substituteEnv(chunk)
+	if subErr != nil {
+		r.err = subErr
+		return 0, subErr
+	}
+
+	n = copy(p, substituted)
+	if n < len(substituted) {
+		// Buffer the rest
+		r.buf = substituted[n:]
+	}
+
+	// Store sticky error for next read
+	if readErr != nil && readErr != io.EOF {
+		r.err = readErr
+	} else {
+		if readErr == io.EOF && n > 0 {
+			return n, nil
+		}
+		if readErr == io.EOF {
+			return n, io.EOF
+		}
+	}
+
+	return n, nil
+}
+
+// findIncompletePatternStart returns the index where an incomplete ${...} pattern starts,
+// or -1 if there's no incomplete pattern at the end.
+func findIncompletePatternStart(data []byte) int {
+	// Look for '$' near the end that might be start of ${VAR}
+	// Maximum var name we reasonably expect + "${}" = ~256 chars
+	searchStart := max(0, len(data)-maxVarNameLength)
+
+	for i := len(data) - 1; i >= searchStart; i-- {
+		if data[i] == '$' {
+			// Check if this is a complete pattern or incomplete
+			if i+1 >= len(data) {
+				// Just "$" at end
+				return i
+			}
+			if data[i+1] == '{' {
+				// Check if there's anything after "${"
+				if i+2 >= len(data) {
+					// Just "${" at end
+					return i
+				}
+				// Check if pattern is complete by looking for '}'
+				for j := i + 2; j < len(data); j++ {
+					if data[j] == '}' {
+						// This pattern is complete, continue searching for another
+						break
+					}
+					if j == len(data)-1 {
+						// Reached end without finding '}', incomplete pattern
+						return i
+					}
+				}
+			}
+		}
+	}
+	return -1
+}