feat: combine mikroorm v6 & v7 implementation

[NaufalK25]

Summary

• Fixes a silent bug: MetadataStorage.getAll() returns a plain Dictionary in MikroORM v6 but a real Map in v7 — the adapter previously assumed the v6 shape (Object.values(...)), which under v7 quietly produced an empty diagram instead of erroring. Now branches on instanceof Map to handle both.
• Adds MikroORM v7 test coverage (test/adapters/mikroorm-v7.test.ts + v7-basic/v7-composite fixtures using @mikro-orm/decorators) in a separate file/CI job from the v6 suite — mixing a v6-broken import into the same worker was observed to corrupt vitest's own error reporting for unrelated tests, not just fail the one that broke.
• New mikroorm-v7 CI job installs @mikro-orm/core@7/@mikro-orm/sqlite@7/@mikro-orm/decorators@7 on top of npm ci (no --save) and runs just the mikroorm-scoped tests with coverage, uploaded to Codecov under its own mikroorm-v7 flag.
• docs/adapters.md's MikroORM section updated to describe both versions and the one real behavioral difference above.

The committed @mikro-orm/core/@mikro-orm/sqlite devDependencies stay pinned to v6 (still what most existing production/NestJS codebases run) — v7 is exercised only via the dedicated CI job swapping the install on the fly, never touching package.json/package-lock.json. Both versions are targeted because the adapter always reads whatever MikroORM version the target project has installed, not orm2erd's own devDependency, so v6-only would silently misbehave for real v7 users and v7-only would break the still-larger v6 user base.

Test plan

• npx vitest run test/adapters/mikroorm.test.ts test/detect/mikroorm.test.ts passes against the committed v6 devDependency
• Manually swapped in @mikro-orm/core@7/@mikro-orm/sqlite@7/@mikro-orm/decorators@7 and confirmed test/adapters/mikroorm-v7.test.ts passes (skips cleanly under v6)
• Full suite (npx vitest run) passes with v6 restored
• npm run lint / npx prettier --check . clean

[codecov-commenter]

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