fix(engine/2.0): stop rejecting valid engine postgres config

[wdawson]

What

Three lines. The storage subtree closed every object with additionalProperties: false; this opens them.

-              "additionalProperties": false
+              "additionalProperties": true

Why not just document the missing fields

That was the first version of this PR, and it was the wrong fix. Enumerating engine config fields here creates a second place to update whenever engine config grows, and the failure mode when it is missed is a hard reject of a config the engine runs happily.

It had already been missed: target_session_attrs has been read by the engine for a while and was absent, and max_conns would have been the second instance in a month. Documenting them fixes today's two and leaves the mechanism that produced them.

The error was also unhelpful. Because the two postgres variants sit inside a oneOf, an unknown key fails every branch, so the message lands on the whole storage node rather than naming the field:

/storage/postgres must have required property 'data_path'
/storage/postgres must NOT have additional properties

This does not weaken the variants

The embedded and external variants are discriminated by required, not by additionalProperties, so opening the objects changes nothing about which one a config matches. Verified with ajv:

Case	Result
example.yaml, embedded variant (regression)	valid
external variant baseline	valid
external + max_conns	valid (was rejected)
external + target_session_attrs	valid (was rejected)
embedded + max_conns	valid
a schedules section of any shape	valid
required key typo'd (passwrod)	still rejected
embedded and external keys together (ambiguous)	still rejected
optional key typo'd (sslmodee)	now accepted — see below

What is given up

Typos in optional keys are no longer caught. Typos in required keys still are, because required catches them independently — so passwrod: p still fails, and only something like sslmodee: disable now slips through to be ignored by the engine.

That is the deliberate trade: the schema stops being a second source of truth that can reject valid config, and in exchange it stops catching one class of typo.

execution_logging still sets additionalProperties: false and is the next likely instance of the same problem. Left alone here rather than widened speculatively.

Context

Consumer side is ArcadeAI/monorepo#3754, which adds max_conns and schedules.max_concurrent_executions to the engine.

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_{— Drake 🐉, Wils's agent}