fix(core): scope requiredness to expressions, and encode + / # per RFC 6570

Addresses Copilot's round-3 review on #2035. It again reported "no new
comments" while carrying two suppressed ones; both reproduced.

1. Requiredness could not live on a variable at all. Deduplication kept the
   first occurrence's group, so in `x{?a}{?b}{a,b}` both names ended up
   required with singleton groups and the form demanded both -- while the
   SDK expands that template with only `a` to "x?a=11". Widening the stored
   group would not have been enough either: in `{a,b}{a,c}`, filling `b` and
   `c` satisfies both expressions, which no per-variable flag can express.

   So requiredness is now returned per expression by `requiredGroups`, and
   `hasRequiredValues` asks that each group be satisfied by any one of its
   names. `TemplateVariable.groupNames` is gone rather than left as a field
   that quietly means something narrower than it reads; `required` remains,
   documented as driving the "Optional" marker and nothing else.

2. `encodeURI` is not the allow-reserved encoder `+` and `#` call for. It
   escapes `[` and `]`, which are reserved and must survive, and it escapes
   `%`, so an already-encoded value is double-encoded. Measured:
   "[::1]" -> "%5B::1%5D" and "%2F" -> "%252F". Both corrupt the URI rather
   than merely over-escaping it -- an IPv6 literal or a pre-encoded path
   reaches the server altered, which is the same class of defect #1919 is
   about.

   Added an RFC 6570 3.2.1 encoder that preserves reserved characters and
   existing pct-triplets, splitting on `%XX` so a lone `%` is still encoded
   to `%25`, and matching with the `u` flag so an astral character is
   encoded whole. `+` and `#` expressions are now taken over even for a
   single name, so both expansion paths agree on what those operators mean.

Signed-off-by: cliffhall <cliff@futurescale.com>

clients/web/src/utils/uriTemplate.ts

@@ -18,6 +18,7 @@ export {
   expandUriTemplate,
   hasRequiredValues,
   parseUriTemplate,
+  requiredGroups,
   templateVariables,
 } from "@inspector/core/mcp/uriTemplate.js";
 export type {