Scope down Go discriminator-method collision detection

The previous change to `goDiscriminatorMethodName` over-broadly flagged
collisions whenever the discriminator property's natural Go name matched the
chosen method name, even on multi-variant unions where the discriminator field
is elided or renamed to `Discriminator` on the emitted variant structs.

The result was 98 unnecessary `Kind()` -> `xxxKind()` method renames across
8 multi-variant unions (PermissionPromptRequest, PermissionDecision,
AgentRegistrySpawnResult, SlashCommandInvocationResult, UserToolSessionApproval,
PermissionsLocationsAddToolApprovalDetails, and the two ApproveFor* variants).

The real natural-name collision case is when the variant struct is a
pre-existing type (already in `ctx.generatedNames`) that retained the
discriminator as a struct field with its natural Go name -- e.g.,
`SessionListEntry`'s `LocalSessionMetadataValue.IsRemote bool` field, which
would clash with an exported `IsRemote() bool` interface method on the union.

This commit narrows the natural-name check to that pre-existing-variant case,
leaving the independent `Discriminator`-rename collision clause unchanged.
Also drops the now-dead `permissionDecisionKind()` helper from the
hand-written `permission_decision_no_result.go`.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: stephentoub

go/rpc/permission_decision_no_result.go

@@ -15,10 +15,6 @@ func (PermissionDecisionNoResult) Kind() PermissionDecisionKind {
 	return PermissionDecisionKind("no-result")
 }
 
-func (PermissionDecisionNoResult) permissionDecisionKind() PermissionDecisionKind {
-	return PermissionDecisionKind("no-result")
-}
-
 // MarshalJSON emits {"kind":"no-result"} for serialization symmetry with
 // the other PermissionDecision variants. The SDK normally suppresses this
 // value before it reaches the wire, but a stable representation is useful