feat: support Responses to Chat (#5787)

* fix(openai): harden Chat-to-Responses compatibility

Add a shared Responses-to-Chat stream state machine and use it from the OpenAI relay path. Preserve assistant text alongside tool calls, bind tool argument deltas by output_index, map incomplete finish reasons, support reasoning/custom tool events, and buffer upstream SSE for non-stream Chat clients.

Add deterministic service tests and relay SSE tests for the conversion path.

Related to #5745.

* refactor: rename openaicompat to relayconvert for improved clarity

* feat(gemini): support responses request conversion

* feat: add responses to chat conversion support

* fix: harden responses chat conversion edge cases

service/relayconvert/regex.go

@@ -0,0 +1,33 @@
+package relayconvert
+
+import (
+	"regexp"
+	"sync"
+)
+
+var compiledRegexCache sync.Map // map[string]*regexp.Regexp
+
+func matchAnyRegex(patterns []string, s string) bool {
+	if len(patterns) == 0 || s == "" {
+		return false
+	}
+	for _, pattern := range patterns {
+		if pattern == "" {
+			continue
+		}
+		re, ok := compiledRegexCache.Load(pattern)
+		if !ok {
+			compiled, err := regexp.Compile(pattern)
+			if err != nil {
+				// Treat invalid patterns as non-matching to avoid breaking runtime traffic.
+				continue
+			}
+			re = compiled
+			compiledRegexCache.Store(pattern, re)
+		}
+		if re.(*regexp.Regexp).MatchString(s) {
+			return true
+		}
+	}
+	return false
+}