Make standalone_nexus_operation capability optional for field presence

[stephanos]

What

Change NamespaceInfo.Capabilities.standalone_nexus_operation from an implicit-presence bool to optional bool.

Why

With a plain proto3 bool, a false value is indistinguishable from an unset field on the wire (both are omitted). This means a DescribeNamespace client cannot tell apart:

1. field absent — server too old to know the capability → not supported
2. field present, false — server supports it, but the dynamic config is disabled
3. field present, true — supported and enabled

Cases 1 and 2 currently collapse into "field omitted". Marking the field optional adds explicit field presence, so a server that knows the capability always emits it (true or false), and only an older server omits it — giving the UI a reliable three-state signal.

Compatibility

• Wire format: compatible — optional bool encodes identically to bool on the same field number.
• Generated Go: source-breaking — the field regenerates as *bool (with a Has...() accessor). Downstream (temporal server) Capabilities{...} literal must wrap the value in proto.Bool(...); getter reads are unaffected.
• buf breaking check: this changes field cardinality (implicit → explicit presence) and will likely trip the breaking-change detector, needing a waiver.

Draft pending discussion on whether to apply the same treatment to the other capability bools.