fix: send an empty-string pagination cursor verbatim on every list adapter (#2220)

An MCP cursor is an opaque string: the spec constrains neither its content
nor its length, so `""` is a `nextCursor` a server may legitimately return
and the client is required to send back verbatim.

`listPrompts`, `listResources`, `listResourceTemplates` and
`listRequestorTasks` built their request params with `cursor ? { cursor } : {}`,
a truthiness check that cannot distinguish "no cursor" from "the cursor is
the empty string". Against a server that paginates with `""` they dropped it
and re-requested page one — a list that silently stops at the first page, or
a walk that loops on it forever. Nothing surfaced an error, because the
request was well-formed; it just asked the wrong question. `listTools` (and
the later `listSkills`) already used `cursor !== undefined`; this brings the
other four into line.

`getReplayableParams` in `protocolReplay.ts` existed partly to mirror the
asymmetry — its `method === "tools/list" || params.cursor !== ""` special
case collapses to `typeof params.cursor === "string"` now that the adapters
agree, and its long comment is updated rather than left describing a state
that no longer exists.

Coverage:

- `inspectorClient-list-cursor.test.ts` asserts the outbound params of all
  five adapters — `""` carried, an absent cursor sending no `cursor` key,
  a non-empty cursor forwarded unchanged. `listTools` is the control.
- `test-servers/configs/empty-cursor-http.json` (`emptyStringCursor`) hands
  out `""` as the cursor for page two, the shape no other fixture produces,
  and `empty-cursor.test.ts` walks all three of its lists end to end.
- Removing the fix from any one adapter fails exactly its two cases.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EN2UfrkQZvtrviMtqURRm4
Signed-off-by: cliffhall <cliff@futurescale.com>

test-servers/configs/empty-cursor-http.json

@@ -0,0 +1,19 @@
+{
+  "serverInfo": {
+    "name": "empty-cursor-showcase",
+    "version": "1.0.0"
+  },
+  "tools": [{ "preset": "numbered_tools", "params": { "count": 12 } }],
+  "resources": [{ "preset": "numbered_resources", "params": { "count": 12 } }],
+  "prompts": [{ "preset": "numbered_prompts", "params": { "count": 12 } }],
+  "maxPageSize": {
+    "tools": 4,
+    "resources": 4,
+    "prompts": 4
+  },
+  "emptyStringCursor": true,
+  "transport": {
+    "type": "streamable-http",
+    "port": 6601
+  }
+}