Remove case-insensitive =?base64? prefix test

The spec states header values are case-sensitive (RFC 9110). The
=?base64? prefix is part of the header value, not the header name,
so it should be matched case-sensitively. This was identified as a
bug in the conformance tests per feedback on SEP-2243.

Changes:
- Remove server-accepts-case-insensitive-base64 check from server
  validation scenario
- Change base64 prefix regex from case-insensitive (/i) to
  case-sensitive in client header validation
- Remove corresponding entry from sep-2243.yaml

Author: Tarek Mahmoud Sayed

src/scenarios/client/http-custom-headers.ts

@@ -38,7 +38,7 @@ const SPEC_REFERENCE_TOOL_DEF = {
  * Base64-encoded values use the format: =?base64?{Base64EncodedValue}?=
  */
 function decodeHeaderValue(value: string): string {
-  const base64Match = value.match(/^=\?base64\?(.+)\?=$/i);
+  const base64Match = value.match(/^=\?base64\?(.+)\?=$/);
   if (base64Match) {
     return Buffer.from(base64Match[1], 'base64').toString('utf-8');
   }
@@ -77,7 +77,8 @@ function validateEncodedHeader(
 ): string | null {
   if (needsBase64Encoding(bodyValue)) {
     // Value requires Base64 encoding
-    const base64Match = rawHeader.match(/^=\?base64\?(.+)\?=$/i);
+    const base64Match = rawHeader.match(/^=\?base64\?(.+)\?=$/);
+
     if (!base64Match) {
       return `Value '${bodyValue}' requires Base64 encoding but header was sent as plain: '${rawHeader}'`;
     }

src/scenarios/sep-2243.yaml

@@ -81,8 +81,7 @@ requirements:
     check: server-rejects-invalid-base64-padding
   - text: 'Servers MUST reject requests with invalid Base64 characters in Mcp-Param values.'
     check: server-rejects-invalid-base64-chars
-  - text: 'Servers MUST accept case-insensitive =?base64? wrappers.'
-    check: server-accepts-case-insensitive-base64
+
   - text: 'Servers MUST return HTTP 400 Bad Request when required standard headers are missing.'
     check: server-rejects-missing-method-header
   - text: 'Servers MUST reject requests where Mcp-Name is omitted but the corresponding body value is present.'

src/scenarios/server/http-standard-headers.ts

@@ -472,7 +472,6 @@ export class HttpCustomHeaderServerValidationScenario implements ClientScenario
 - Server MUST validate Base64-encoded header values
 - Server MUST reject requests with invalid Base64 padding or characters
 - Server MUST treat values without =?base64?...?= wrapper as literal
-- Server MUST accept case-insensitive =?base64? prefix
 - Server MUST reject requests where custom header is omitted but value is in body`;
 
   async run(serverUrl: string): Promise<ConformanceCheck[]> {
@@ -696,25 +695,6 @@ export class HttpCustomHeaderServerValidationScenario implements ClientScenario
         defaultHeaders
       );
 
-      // Case-insensitive Base64 prefix - server MUST accept
-      await this.testBase64Case(
-        checks,
-        serverUrl,
-        baseHeaders,
-        nextId,
-        'accept',
-        'server-accepts-case-insensitive-base64',
-        'ServerAcceptsCaseInsensitiveBase64',
-        'Server MUST accept case-insensitive =?BASE64? prefix',
-        xMcpTool.name,
-        paramName,
-        'Hello',
-        headerSuffix,
-        `=?BASE64?${validBase64Value}?=`,
-        defaultArgs,
-        defaultHeaders
-      );
-
       // --- Missing Custom Header with Value in Body ---
 
       await this.testMissingCustomHeader(