sqlparser: reject empty migration context at parse time for all CONTEXT variants

All CONTEXT grammar rules now call yylex.Error + return 1 when the
context string is empty, matching the behaviour added for CANCEL CONTEXT
in the base branch.  Empty-context round-trip tests are removed from the
valid-SQL table and replaced with entries in invalidSQL that assert the
exact error message.  Executor doc comments no longer mention the
"empty context is equivalent to ALL" caveat that is now impossible to
reach.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Shlomi Noach <2607934+shlomi-noach@users.noreply.github.com>

Author: shlomi-noach

go/vt/sqlparser/parse_test.go

@@ -2660,9 +2660,6 @@ var validSQL = []struct {
 	input: "alter vitess_migration cleanup all",
 }, {
 	input: "alter vitess_migration cleanup context 'some-context'",
-}, {
-	input:  "alter vitess_migration cleanup context ''",
-	output: "alter vitess_migration cleanup all",
 }, {
 	input: "alter vitess_migration '9748c3b7_7fdb_11eb_ac2c_f875a4d24e90' launch",
 }, {
@@ -2673,9 +2670,6 @@ var validSQL = []struct {
 	input: "alter vitess_migration launch all",
 }, {
 	input: "alter vitess_migration launch context 'some-context'",
-}, {
-	input:  "alter vitess_migration launch context ''",
-	output: "alter vitess_migration launch all",
 }, {
 	input: "alter vitess_migration '9748c3b7_7fdb_11eb_ac2c_f875a4d24e90' complete",
 }, {
@@ -2686,27 +2680,18 @@ var validSQL = []struct {
 	input: "alter vitess_migration complete all",
 }, {
 	input: "alter vitess_migration complete context 'some-context'",
-}, {
-	input:  "alter vitess_migration complete context ''",
-	output: "alter vitess_migration complete all",
 }, {
 	input: "alter vitess_migration '9748c3b7_7fdb_11eb_ac2c_f875a4d24e90' postpone complete",
 }, {
 	input: "alter vitess_migration postpone complete all",
 }, {
 	input: "alter vitess_migration postpone complete context 'some-context'",
-}, {
-	input:  "alter vitess_migration postpone complete context ''",
-	output: "alter vitess_migration postpone complete all",
 }, {
 	input: "alter vitess_migration '9748c3b7_7fdb_11eb_ac2c_f875a4d24e90' cancel",
 }, {
 	input: "alter vitess_migration force_cutover all",
 }, {
 	input: "alter vitess_migration force_cutover context 'some-context'",
-}, {
-	input:  "alter vitess_migration force_cutover context ''",
-	output: "alter vitess_migration force_cutover all",
 }, {
 	input: "alter vitess_migration '9748c3b7_7fdb_11eb_ac2c_f875a4d24e90' force_cutover",
 }, {
@@ -2738,19 +2723,10 @@ var validSQL = []struct {
 	input: "alter vitess_migration throttle context 'some-context' ratio 0.7",
 }, {
 	input: "alter vitess_migration throttle context 'some-context' expire '1h' ratio 0.7",
-}, {
-	input:  "alter vitess_migration throttle context ''",
-	output: "alter vitess_migration throttle all",
-}, {
-	input:  "alter vitess_migration throttle context '' expire '1h' ratio 0.7",
-	output: "alter vitess_migration throttle all expire '1h' ratio 0.7",
 }, {
 	input: "alter vitess_migration unthrottle all",
 }, {
 	input: "alter vitess_migration unthrottle context 'some-context'",
-}, {
-	input:  "alter vitess_migration unthrottle context ''",
-	output: "alter vitess_migration unthrottle all",
 }, {
 	input: "alter vitess_migration throttle all expire '1h'",
 }, {
@@ -6536,6 +6512,30 @@ var invalidSQL = []struct {
 }{{
 	input:  "alter vitess_migration cancel context ''",
 	output: "migration context cannot be empty at position 41",
+}, {
+	input:  "alter vitess_migration cleanup context ''",
+	output: "migration context cannot be empty at position 42",
+}, {
+	input:  "alter vitess_migration launch context ''",
+	output: "migration context cannot be empty at position 41",
+}, {
+	input:  "alter vitess_migration complete context ''",
+	output: "migration context cannot be empty at position 43",
+}, {
+	input:  "alter vitess_migration postpone complete context ''",
+	output: "migration context cannot be empty at position 52",
+}, {
+	input:  "alter vitess_migration force_cutover context ''",
+	output: "migration context cannot be empty at position 48",
+}, {
+	input:  "alter vitess_migration throttle context ''",
+	output: "migration context cannot be empty at position 43",
+}, {
+	input:  "alter vitess_migration throttle context '' expire '1h' ratio 0.7",
+	output: "migration context cannot be empty at position 65 near '0.7'",
+}, {
+	input:  "alter vitess_migration unthrottle context ''",
+	output: "migration context cannot be empty at position 45",
 }, {
 	input:  "select : from t",
 	output: "syntax error at position 9 near ':'",