Make GraphQLRequestHandler extensible via virtual members

[xperiandri]

Turn the introspection/operation/dispatch steps of GraphQLRequestHandler from private let-bound closures into abstract/default members, so a derived handler can override a single step (e.g. per-request introspection, or custom operation execution) without reimplementing the whole class:

• ExecuteIntrospectionQuery and ExecuteOperation are now virtual, each resolving options.CurrentValue.SchemaExecutor itself instead of receiving it as a parameter (dropping the now-pointless generic <'Root> from ExecuteOperation, which used to shadow the class's own 'Root without being constrained by it).
• CheckOperationType becomes a public member (was a private local function), with a <remarks> note that it consumes the request body, so calling it more than once on the same request fails to bind on the second call.
• HandleAsync is now virtual too, and dispatches to the other members through handler instead of closing over local functions.

Also move the class-level <summary> from the primary constructor's doc comment to a /// line on the type itself, and give the constructor its own "Initializes a new instance..." comment - the constructor comment documented the class, not the constructor.

[Copilot]

🔵 Needs a closer look

Add coverage confirming HandleAsync dispatches through the overridable members.

Pull request overview

This PR makes GraphQLRequestHandler extensible through overridable request-processing members.

Changes:

• Adds virtual introspection, operation execution, and handling steps.
• Exposes CheckOperationType and documents request-body consumption.
• Updates type and constructor XML documentation.

File summaries

File	Summary	Review note
src/FSharp.Data.GraphQL.Server.AspNetCore/GraphQLRequestHandler.fs	Adds virtual handler extension points and updated documentation.	Moderate: add tests verifying derived-handler overrides are reached.

Review details

Suppressed comments (1)

src/FSharp.Data.GraphQL.Server.AspNetCore/GraphQLRequestHandler.fs:261

• The new virtual extension points and the handler.* dispatch are not covered by tests; the existing ASP.NET Core tests only exercise serialization/websocket messages. Please add an xUnit test with a derived handler overriding the execution/dispatch members and assert that HandleAsync reaches those overrides, otherwise this central extensibility contract can regress back to closed-over implementations unnoticed.

            match! handler.CheckOperationType () with
            | IntrospectionQuery optionalAstDocument -> return! handler.ExecuteIntrospectionQuery optionalAstDocument
            | OperationQuery content -> return! handler.ExecuteOperation (content)

• Files reviewed: 1/1 changed files
• Comments generated: 0
• Review effort level: Lite

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[Copilot]

🟢 Approval recommended

No unresolved blocking issues were identified.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 0 new
• Review effort level: Lite

[github-actions]

Test Results

    9 files      9 suites   11m 10s ⏱️
  710 tests   705 ✅  5 💤 0 ❌
2 130 runs  2 115 ✅ 15 💤 0 ❌

Results for commit b5cc7d1.