feat(templates): add toJson helper for object claims (#165)

Header templates can now emit an object or array claim as one JSON
header. Example: {{get .Claims "tenant" | toJson}}. The backend parses
the claim, so the middleware does not need to know its schema.

The helper adds no new access path. The validator treats toJson like
default and checks its arguments. A claim must be on the whitelist, and
a dump of the whole claims map is rejected. An absent or empty claim
skips the header. The JSON encoder escapes control characters.

A rendered header value over 8192 bytes is still dropped. The drop is
now logged at error level, with the header name and without the value.
Other sanitizer drops stay at debug level.

Tests cover value types, edge cases, whitelist bypass attempts, token
leaks and header injection. A new yaegi check runs the helper under the
interpreter. README, configuration docs and the plugin manifest show the
new helper.

bearer_auth.go

@@ -223,6 +223,9 @@ func headerInjectionRuneReason(r rune) string {
 	return ""
 }
 
+// headerReasonTooLong is the headerValueReason result for an over-long value.
+const headerReasonTooLong = "exceeds max length"
+
 // headerValueReason reports why value is unsafe to forward as a free-form HTTP
 // header value, or "" if acceptable. It rejects values over maxLen (maxLen<=0
 // disables the check) and values containing control or bidi-override runes, but
@@ -230,7 +233,7 @@ func headerInjectionRuneReason(r rune) string {
 // never includes the value, so it is safe to log.
 func headerValueReason(value string, maxLen int) string {
 	if maxLen > 0 && len(value) > maxLen {
-		return "exceeds max length"
+		return headerReasonTooLong
 	}
 	for _, r := range value {
 		if reason := headerInjectionRuneReason(r); reason != "" {