GraphQL combined schema with id-keyed top-level Query fields

[gilesknap]

Parent

#351

What to build

The GraphQL transport synthesises one combined schema with N top-level Query fields, each keyed by its controller id. Clients query all configured devices from a single endpoint.

Scope:

• GraphQL transport adopts the multi-controller connect() properly: one schema, one top-level Query field per id.
• Per-transport GraphQL id validator runs at connect() time. GraphQL has the most restrictive identifier charset and so drives the lowest-common-denominator note in the docs (cross-transport id-naming guidance for users mixing EPICS hyphens with GraphQL field names).
• End-to-end multi-controller GraphQL scenario added covering: two controllers exposed under a single combined schema with distinct id-keyed Query fields.
• GraphQL transport test gains a per-transport id-validation case demonstrating the fail-fast behaviour.
• Documentation note added about id charset constraints when mixing transports (lowest-common-denominator).

User stories from #351 covered: 16, 17, 18, 20.

Acceptance criteria

• GraphQL endpoint exposes one combined schema with one top-level Query field per controller id
• GraphQL id validator rejects illegal characters with a clear startup error
• Multi-controller GraphQL scenario test passes
• GraphQL transport test covers id-validation fail-fast
• Lowest-common-denominator id-naming guidance added to docs
• Full test suite stays green

Blocked by

• Multi-controller foundation end-to-end via REST transport #353