fix: narrow the exemption per transport (#2319 round 18)

Round 17 widened exemptMcpEndpoint to the origin for everything, which
exempted same-origin OAuth work as well as MCP traffic — on the very
401/discovery path #2319 exists to bound.

The rule is now as narrow as each transport allows. Streamable HTTP
sends every MCP request to the configured URL, so origin + pathname is
exact there and same-origin OAuth stays bounded. Legacy SSE negotiates
its message endpoint inside the stream, where we never see it, so the
origin rule is the narrowest available — and it is exact rather than
approximate, since the SDK enforces that the message endpoint shares
the stream URL's origin.

The residual gap is therefore confined to legacy SSE against a server
hosting its own OAuth endpoints, and it is stated in the docblock and
pinned by a test rather than left implicit. Neither alternative was
reachable: the SDK takes a single fetch for all network requests, so
there is no separate auth seam to route through, and the negotiated SSE
endpoint is parsed inside the SDK's own stream handler.

Fails open on every uncertainty, including not knowing the transport —
a path rule applied to what turns out to be SSE severs real MCP
traffic, so an absent or throwing hint falls back to the origin rule.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YFnM6RqB4nXqNysgUYmjmq
Signed-off-by: cliffhall <cliff@futurescale.com>

core/mcp/inspectorClient.ts

@@ -840,7 +840,14 @@ export class InspectorClient extends InspectorClientEventTarget {
       withOAuthRequestTimeout(
         withOverrides,
         DEFAULT_OAUTH_REQUEST_TIMEOUT_MS,
-        exemptMcpEndpoint(() => this.getServerUrl()),
+        exemptMcpEndpoint(
+          () => this.getServerUrl(),
+          // Legacy SSE negotiates its message endpoint inside the stream, so
+          // its path is unknowable here and the exemption has to widen to the
+          // origin. Streamable HTTP sends everything to the configured URL, so
+          // the path rule is exact there and same-origin OAuth stays bounded.
+          () => this.transportConfig?.type === "sse",
+        ),
       ),
     );
     // #2319: the auth chain is composed separately so the deadline sits