Clarify DEFAULT_GENERATED vs ON UPDATE distinction with test coverage

[Copilot]

In response to feedback questioning why DEFAULT_GENERATED shouldn't be treated as a computed column alongside ON UPDATE CURRENT_TIMESTAMP.

Key Distinction

MySQL's EXTRA column contains separate indicators:

• DEFAULT_GENERATED (from DEFAULT CURRENT_TIMESTAMP) - provides a default value but can be overridden during INSERT
• ON UPDATE CURRENT_TIMESTAMP - automatically updates on every UPDATE regardless of user input

Only the latter exhibits true computed/read-only behavior.

Changes

• Test coverage: Added TimestampComputedColumnTests.cs demonstrating:
  • Explicit override capability of DEFAULT CURRENT_TIMESTAMP columns during INSERT
  • Automatic update behavior of ON UPDATE CURRENT_TIMESTAMP columns
  • Current implementation correctly marks only ON UPDATE columns as IsGenerated

Example test table structure:

CREATE TABLE TimestampTestTable (
    CreatedAt TIMESTAMP DEFAULT CURRENT_TIMESTAMP,              -- IsGenerated = false
    UpdatedAt TIMESTAMP DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,  -- IsGenerated = true
    OnUpdateOnly TIMESTAMP NULL ON UPDATE CURRENT_TIMESTAMP     -- IsGenerated = true
)

Treating DEFAULT_GENERATED as computed would incorrectly prevent applications from specifying explicit timestamps during inserts.

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[rhuijben]

Code integrated via other pr.