feat: add ListMysqlDatabases endpoint

[VoLKyyyOG]

Create new endpoint for use by metanexus.
This lists all mysql databases.

ref: https://linear.app/squareup/issue/DFNS-445/create-schemabot-endpoint-listmysqldatabases

[Copilot]

Pull request overview

Adds a new API endpoint for listing configured MySQL databases from SchemaBot’s server configuration.

Changes:

• Defines response types for MySQL database inventory.
• Registers and implements GET /api/mysql/databases with optional environment filtering.
• Adds handler tests and updates API README route documentation.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
pkg/apitypes/apitypes.go	Adds response structs for MySQL database inventory.
pkg/api/service.go	Registers the new MySQL database listing route.
pkg/api/README.md	Documents the new route and handler file.
pkg/api/mysql_handlers.go	Implements the MySQL database inventory handler and filtering logic.
pkg/api/mysql_handlers_test.go	Adds tests for listing, filtering, and empty configuration behavior.

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