fix: insert VALUES between-values hint points at comma not close-paren

The ambient-hint fallback in ambient_hint_core_in_mode parsed
schemalessly, so the type-blind grammar closed an `insert … values
(…)` tuple after the first value and the "Next:" hint pointed at `)`.
With a schema available the walk knows the remaining columns and the
correct next token is `,`. Parse the fallback with the schema cache so
the expected-token prose matches the rest of the (already
schema-aware) hint ladder.

Also corrects wrong-arity closed tuples where the schemaless parse
accepted the input and the hint said "submit with Enter" for a command
the schema-aware parse rejects — the hint now surfaces the accurate
error. Three typing-surface snapshots updated to match.

Docs: ADR-0022 Amendment 3 (+ README index) records the schema-aware
fallback; requirements.md H1a cites the hint-accuracy improvement.

tests/typing_surface/snapshots/typing_surface_matrix__typing_surface__insert_form_a__form_a_in_progress_with_one_value_is_incomplete@form_a_in_progress_one_value.snap

@@ -1,5 +1,6 @@
 ---
 source: tests/typing_surface/insert_form_a.rs
+assertion_line: 243
 description: "input=\"insert into Orders (OrderId, CustId, Total) values (42\" cursor=54"
 expression: "& a"
 ---
@@ -9,7 +10,7 @@ Assessment {
     state: IncompleteAtEof,
     hint: Some(
         Prose(
-            "Next: `)`",
+            "Next: `,`",
         ),
     ),
     completion: None,

tests/typing_surface/snapshots/typing_surface_matrix__typing_surface__insert_form_b__form_b_with_too_few_values_is_invalid_at_close_paren@form_b_too_few_values.snap

@@ -1,5 +1,6 @@
 ---
 source: tests/typing_surface/insert_form_b.rs
+assertion_line: 168
 description: "input=\"insert into Customers values ('Alice')\" cursor=38"
 expression: "& a"
 ---
@@ -11,7 +12,7 @@ Assessment {
     ),
     hint: Some(
         Prose(
-            "Submit with Enter",
+            "after `insert into Customers values ('Alice'`, expected `,` — usage: insert into <Table> [(<col>[, ...])] [values] (<value>[, ...])",
         ),
     ),
     completion: None,

tests/typing_surface/snapshots/typing_surface_matrix__typing_surface__insert_form_c__form_c_wrong_value_count_is_invalid@form_c_wrong_count.snap

@@ -1,5 +1,6 @@
 ---
 source: tests/typing_surface/insert_form_c.rs
+assertion_line: 101
 description: "input=\"insert into Customers ('Alice', 'a@b.c', 'extra')\" cursor=49"
 expression: "& a"
 ---
@@ -11,7 +12,7 @@ Assessment {
     ),
     hint: Some(
         Prose(
-            "Submit with Enter",
+            "after `insert into Customers ('Alice', 'a@b.c'`, expected `)` — usage: insert into <Table> [(<col>[, ...])] [values] (<value>[, ...])",
         ),
     ),
     completion: None,