[Fusion] Add conditions to plan nodes

[tobias-tengler]

If either all selections on the root level or below a lookup are conditional, we can pull out those conditions directly onto the execution node and skip execution entirely, if the conditions are not met.

For example for this operation

query testQuery($skip: Boolean!) {
  productBySlug(slug: "product") @skip(if: $skip) {
    name
  }
}

we can generate this execution node

- id: 1
  type: Operation
  schema: A
  operation: |
    query testQuery_60827aef_1 {
      productBySlug(slug: "product") {
        name
      }
    }
  conditions:
    - variable: $skip
      passingValue: false

This PR does not yet account for all edge cases that come with this.
For a fully functional solution we'll also have to rewrite the SelectionSetPartitioner as it's currently not preserving conditional inline fragments around selections.

[Copilot]

Pull Request Overview

This PR adds support for conditions to execution plan nodes in the Fusion GraphQL system. The changes enable the execution planner to extract conditional directives (@Skip and @include) from GraphQL operations and represent them as execution node conditions, allowing for optimized conditional execution.

Key changes:

• Added ExecutionNodeCondition class to represent conditional directives
• Extended execution nodes with condition support and evaluation logic
• Updated execution planner to extract and process conditional selections
• Modified serialization/deserialization to handle node conditions

Reviewed Changes

Copilot reviewed 77 out of 77 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
Test snapshot files	Updated to reflect new condition and forwardedVariables fields in execution plans
ConditionalTests.cs	Added comprehensive test cases for conditional directive handling across different scenarios
WorkItem.cs	Changed visibility from public to internal for planning components
OperationPlanStep.cs	Added Conditions property and Lookup field to plan steps
NodeFieldPlanStep.cs	Added Conditions property and renamed from NodePlanStep
Various planning files	Updated to support condition extraction and node field work items
ExecutionNode classes	Added condition support with evaluation logic in base ExecutionNode
Serialization classes	Updated formatters and parsers to handle conditions in JSON/YAML output

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