fix(prisma): Prisma.Database with omitted branch attaches the project default branch

[kristof-siket]

Summary

The three Prisma resources that take an optional branch attachment interpret the same omission three different ways:

Resource	branchId/branchGitName omitted means
Prisma.App	resolve the project's default branch client-side and attach (desiredBranchId; fails loudly when the project has no default branch)
Prisma.Bucket	pass through; the Management API attaches the default branch server-side
Prisma.Database	desired state is UNASSIGNED — actively detach

The Prisma.Database reading is the odd one out, and it bites in two ways:

• a database attached to a branch out-of-band (Console, CLI, another tool) gets actively detached on every reconcile, because branchNeedsSync returns database.branchId !== null when both props are omitted, and the reconcile patch path maps omission to an explicit { branchId: null } PATCH;
• databases created via Prisma.Database with no branch props show up as "Unassigned" in the Prisma Console.

Downstream consumers currently work around the asymmetry by always passing branchId explicitly (see prisma/composer#258).

What changes

Prisma.Database now treats an omitted branch exactly like Prisma.App: resolve the project's default branch and converge toward it. branchId: null is never emitted.

• New defaultBranchIdOf + resolvedBranchAttachment helpers mirror App's desiredBranchId, including the same-shaped errors for a missing or ambiguous default branch ("has no default branch to attach database … Create or promote a default branch, or specify branchId/branchGitName").
• branchNeedsSync's omitted arm compares the observed branchId against the resolved default branch id instead of !== null.
• The reconcile patch path writes the resolved attachment — the { branchId: null } detach PATCH is gone.
• Generated-name creates pass the resolved default branchId in the create body, so a fresh database is born attached.
• DatabaseProps.branchId/branchGitName lose | null; runtime null is rejected in validateDatabaseProps and an early diff guard with a clear error (mirroring validateAppProps), rather than silently resolving to the default — a caller who meant "detach" gets a loud contract error.
• The existing refusal of explicit name + branch attachment at create is kept verbatim (the Management API still creates before attaching with no idempotency key).

Behavior notes

• Pre-existing unassigned databases converge: one in-place PATCH onto the default branch on the next reconcile — an update, never a replace. A database already attached to the default branch is left untouched.
• Explicitly named creates (branch omitted) are still created project-scoped (the atomicity refusal above), then converge onto the default branch via PATCH in the same reconcile.
• Breaking edge: explicit branchId: null / branchGitName: null — previously the only way to request detachment — now fails validation. Anyone who deliberately wanted an unassigned database loses that expressibility; an unassigned database is no longer representable as desired state, matching Prisma.App.

Tests

New test/Prisma/Database.test.ts (fake-client provider tests in the App.test.ts style) pins:

• a generated-name create resolves the default branch and carries it in the create body (no follow-up PATCH);
• an attached database with omitted props is not detached — updateDatabase dies if called (regression test for the old behavior);
• a pre-existing unassigned database converges onto the default branch with exactly one PATCH, and diff plans an update, not a replace;
• an explicit branchId stays authoritative (no default-branch lookup);
• explicit null branch props are rejected at reconcile and diff;
• a project with no default branch fails with the clear, App-shaped error;
• an explicitly named create converges onto the default branch via PATCH in the same reconcile.

Existing suites updated to the new contract: the old "detaches an observed branch when branch props are omitted" regression test is inverted into "keeps the observed branch attachment", and the fake Management clients in Resources.test.ts / ManagementLifecycle.test.ts gained listBranches / branch-aware createDatabase/updateDatabase.

pnpm test test/Prisma — 376 passed, 0 failed. pnpm tsc -b and pnpm docs:check-jsdoc clean.

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