docs: say "safe integer" for progressToken, matching what the code enforces

The copy said `progressToken` may be "an integer" while the implementation
validates with `ProgressTokenSchema` — zod's `.int()`, which rejects anything
past `Number.MAX_SAFE_INTEGER`. This repo's own tests assert that
`MAX_SAFE_INTEGER + 1` is dropped, so the documentation was promising a value
the client discards.

Corrected in all three places, so the code comment, the docs callout and the UI
hint agree: `dropInvalidReservedMeta`'s doc block (which review did not flag),
the `metadata` reserved-keys callout, and the Request Metadata hint.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0177mPHAdECD18nLLCTwR5rh
Signed-off-by: cliffhall <cliff@futurescale.com>

clients/web/src/components/groups/ServerSettingsForm/ServerSettingsForm.tsx

@@ -734,7 +734,8 @@ export function ServerSettingsForm({
               objects, arrays, numbers, booleans, <Code>null</Code> — not just
               strings. Leave it as <Code>{"{}"}</Code> to send none. The keys
               the protocol reserves keep their own shape:{" "}
-              <Code>progressToken</Code> must be a string or an integer, and{" "}
+              <Code>progressToken</Code> must be a string or a safe integer (one
+              JavaScript can represent exactly), and{" "}
               <Code>io.modelcontextprotocol/related-task</Code> an object with a
               string <Code>taskId</Code>. A reserved key that does not match is
               dropped from the request rather than sent.