Replaced failwith in query planning with specific exceptions

[xperiandri]

Summary

Query planning (Planning.fs) runs after validation, so its error branches are meant to be unreachable. They still happen when validation has a gap, when a schema uses type definition implementations the planner does not handle, or when the planner itself has a bug — "Expected an Abstraction!" has already surfaced in practice (see the regression comments in PlanningTests.fs). Until now all of them threw a bare System.Exception with messages that did not name the field or type involved.

This PR replaces every failwith in Planning.fs with a specific exception type and a descriptive message.

Situation	Before	After
Field not defined in the object definition	Exception	MalformedGQLQueryException
Non-integer @stream argument	Exception	MalformedGQLQueryException, names the argument, field and provided value
Mutation or subscription without a root object in the schema	Exception	MalformedGQLQueryException, same message as the check in Executor
Object definition implementation not supported by planning	Exception "Unexpected parentdef type!"	NotSupportedException, names the definition and its implementation type
Return type definition not supported by planning	Exception "Invalid Return Type in Planning!"	NotSupportedException, names the field, the definition and its implementation type
Fragment planned with an unexpected execution kind	Exception "Expected a Selection!" / "Expected an Abstraction!"	InvalidOperationException, names the expected and actual kind
Merging a field planned with different execution kinds	Exception	InvalidOperationException, names the field and both kinds
Operation root not planned as SelectFields	Exception with the whole planned subtree printed via %A	InvalidOperationException, names the actual kind
Streamed field without a @stream directive	Exception	InvalidOperationException

Notes:

• MalformedGQLQueryException is already used in this file for invalid variable types, carries the Validation error kind and implements IGQLError.
• A private kindName helper prints only the name of an ExecutionInfoKind case instead of the whole planned subtree.
• The subscription message no longer says "no mutation type defined".
• Debug.Fail calls are kept as they were.

Merge conflicts with #598

Both PRs change Planning.fs and append to the 4.0.0 section of RELEASE_NOTES.md, so whichever is merged second needs a small rebase:

• getStreamBufferMode: Added Define.TaskSeqField with streaming of IAsyncEnumerable results #598 switches it to List.vtryFind and ValueSome/ValueNone, this PR changes the exception in the same function. Keep both.
• RELEASE_NOTES.md: keep both entries.

Testing

• No test reaches these branches through the public API because validation rejects such queries first, and Debug.Fail would terminate a Debug test run. No new tests are added.
• On SDK 10.0.303: FSharp.Data.GraphQL.Server builds in Release with warnings as errors; unit tests 596 passed, 0 failed, 5 skipped (already skipped on dev).

🤖 Generated with Claude Code

[Copilot]

🟢 Approval recommended

The exception classifications and messages consistently match their respective validation, unsupported-type, and internal-planner failure cases.

Pull request overview

Replaces generic query-planning failures with meaningful exception types and diagnostic messages.

Changes:

• Adds descriptive exceptions for malformed queries, unsupported definitions, and planner invariant failures.
• Adds concise execution-kind formatting.
• Documents the behavioral change in release notes.

File summaries

File	Description
src/FSharp.Data.GraphQL.Server/Planning.fs	Introduces specific planning exceptions and clearer diagnostics.
RELEASE_NOTES.md	Records the exception-handling change.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Balanced

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Test Results

    3 files      3 suites   10m 13s ⏱️
  601 tests   596 ✅  5 💤 0 ❌
1 803 runs  1 788 ✅ 15 💤 0 ❌

Results for commit 572294c.