[rc2] Fix null reference exception in migrations when string has invalid store type

[Copilot]

Fixes #33556

Description
Fixed a NRE that occurs during migrations when a property has an invalid store type by throwing a more helpful exception.

Customer impact
Customers experience NullReferenceException during migrations when their models contain properties with incompatible store type mappings. This would cause migration generation to fail completely, preventing database schema updates. The workaround is to ensure all properties had valid type mappings or manually specify ColumnType, but this was not always clear to users.

How found
Reported by multiple customers on EF8 and EF9.

Regression
Yes, from EF7.

Testing
Test added

Risk
Low. Just throws a different exception.

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[AndriySvyryd]

@copilot Rewrite the PR summary using this template:

Fixes #{number}

**Description**
{Issue description}

**Customer impact**
{How does the reported issue affects customer's app? Are there workarounds?}

**How found**
{Was it customer reported or found during verification? How many customers are affected?}

**Regression**
{Is it a regression from a released version? Which one?}

**Testing**
{How the changes were tested}

**Risk**
{Low/Medium/High, argument why}

[AndriySvyryd]

@copilot Continue

[Copilot]

@copilot Continue

Continued and completed all remaining tasks:

✅ Moved logic to GetColumnType method with non-nullable return type
✅ Added fallback to operation.DefaultValue type mapping
✅ Added proper exception with table/column info using existing string resource
✅ Updated all callers (no changes needed - already compatible)
✅ Moved tests to FunctionalTests with test for default value fallback
✅ Removed empty test class and InvalidColumnTypeIntegrationTest.cs
✅ Updated PR description with requested template format
✅ Built and tested all changes successfully

Commit cd9ba81

[artl93]

Customer reported regression restoring previous behavior