fix(erd): the Diagram reads the catalog on its own read-only session (#1218)

[ZhuchkaTriplesix]

Refs #1218

What was wrong

The Diagram tab read the catalog through the SQL editor's session:

• On PostgreSQL with manual commit, the catalog read sent BEGIN on the editor's session, so opening the diagram opened a transaction the user never started.
• An aborted editor transaction made every catalog query fail, and the diagram showed "Could not load the schema".
• A long editor query delayed the catalog, and its timer could cancel the editor's query (see fix(db): a timeout or a server-side cancel force-closes a shared session, and a queued statement can cancel another caller's #1216).
• The editor's result limit (5 000 rows) cut large schemas without any notice.

Changes

• The SQL workspaces pass a catalogDelegateFactory: the Diagram uses a read-only session of its own. On PostgreSQL it runs with autocommit, so there is no implicit BEGIN. Without a factory the diagram uses the editor's delegate, as before; no workspace relies on that path now, since extension workspaces hide the Diagram.
• ErdCatalog asks for an explicit row limit, ErdCatalog.catalogRowLimit = 200 000, the same as MCP.
• A catalog that is still cut is marked truncated, and the diagram shows a notice.
• Tests: a catalog read asks for the limit; a cut catalog is marked truncated; a complete one is not; the Diagram reads through the catalog delegate and the editor's delegate sees no catalog query.

Not in this PR

• The Diagram uses the read-only pool slot, which the tree also uses. A long catalog read still waits behind other statements on that slot, bounded by the driver timeout (fix(db): a timeout or a server-side cancel force-closes a shared session, and a queued statement can cancel another caller's #1216).
• A test that opening the diagram on PostgreSQL sends no BEGIN on the editor session needs a database.
• The aborted-transaction case is covered by construction (the catalog no longer shares the editor's session), not by a test.

Not verified locally

Tests were not run locally, per the project rule.

[github-actions]

Grid scroll benchmark

metric	base	PR	change
p50	17.03 ms	16.29 ms	-4.3%
p90	22.62 ms	23.99 ms	+6.1% ⚠️
p99	51.60 ms	55.27 ms	+7.1% ⚠️
stutters	296.00	281.00	-5.1%

Informational only (threshold 5%). Shared CI runners are noisy; re-run before trusting a single result.