[rc2] Fix AmbiguousMatchException when migration classes inherit DbContextAttribute

[Copilot]

Fixes #35590

Description
AmbiguousMatchException is thrown when migration classes inherit from each other and both have DbContextAttribute. The issue occurs because GetCustomAttribute<DbContextAttribute>() finds multiple attributes in the inheritance chain when using the default behavior that includes inherited attributes.

Customer impact
Customers who create extended migrations that inherit from other migration classes and apply DbContextAttribute to both the base and derived classes encounter a runtime exception during migration operations. This prevents normal migration operations from working.

The workaround is to avoid using inheritance for migration classes (this isn't a common pattern) or to not apply DbContextAttribute to base migration classes, but this limits code reuse.

How found
Customer reported on EF9.

Regression
Yes, from EF8.

Testing
Test added.

Risk
Low. The change is surgical and only affects the specific attribute lookup behavior. This doesn't change any existing functionality for non-inheritance scenarios.

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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 Rewrite the PR summary using this template:

...

Rewrote the PR description using the requested template format, including sections for Description, Customer impact, How found, Regression, Testing, and Risk assessment. The updated description provides comprehensive details about the fix and its impact.

@copilot Continue

Completed both requested changes:

1. Rewrote the PR description using the template format with all required sections
2. Fixed the test by updating the assertion from 2 to 3 migrations to account for the new BaseMigration class

All tests are now passing and the changes have been committed.

[artl93]

Customer reported regression straightforward fix