test(analyzer): add PX1121 MissingSchemaMutationGuard test coverage

[kbibelhausen]

Summary

Adds test coverage for the PX1121 MissingSchemaMutationGuard analyzer introduced alongside the production code. No production code was modified.

Positive cases (diagnostic expected at line 9, col 4)

Test	Scenario
Diagnostic_BareAlterTableAdd	ALTER TABLE … ADD with no guard
Diagnostic_MultipleUnguardedMutations	Two bare ALTER TABLE … ADD statements
Diagnostic_UnrelatedGuardDoesNotSatisfyMutation	An IF EXISTS … PRINT guard that does not cover the ALTER TABLE
Diagnostic_ConstantStringConcatenation	String-concat form: "ALTER TABLE SOOrder " + "ADD …"
Diagnostic_InterpolatedIdentifiers	Interpolated form: $"ALTER TABLE {tableName} ADD {columnName} …" (line 11, col 4)
Diagnostic_BareCreateTable	CREATE TABLE with no OBJECT_ID/IF NOT EXISTS guard
Diagnostic_BareCreateIndex	CREATE INDEX with no INDEXPROPERTY guard

Negative cases (no diagnostic expected)

Test	Guard pattern exercised
NoDiagnostic_ColLengthGuard	IF COL_LENGTH(…) IS NULL before ALTER TABLE … ADD
NoDiagnostic_IfNotExistsSysColumns	IF NOT EXISTS (SELECT 1 FROM sys.columns …)
NoDiagnostic_ObjectIdGuardCreateTable	IF OBJECT_ID(…, 'U') IS NULL before CREATE TABLE
NoDiagnostic_IndexPropertyGuardCreateIndex	IF INDEXPROPERTY(OBJECT_ID(…), …, 'IndexID') IS NULL before CREATE INDEX
NoDiagnostic_BeginEndBlockUnderGuard	IF COL_LENGTH … BEGIN … END block
NoDiagnostic_NonSchemaMutationPlugin	INSERT only — not a schema mutation
NoDiagnostic_AlterInCommentIsIgnored	ALTER appears only inside -- line comment, /* */ block comment, and a string literal

Notes

• All Sources files use using PX.Data; only; no PX.Web.Customization reference required because the analyzer fires on any PXDatabase.Execute call regardless of enclosing class.
• Build verification deferred to upstream Windows CI (ExternalDependency.csproj targets net48, unavailable on macOS).
• Codex review: CLEAR TO SHIP — no actionable issues found.