feat(jit): add operation plan document conversion

[jaxassistant55]

Summary:
Adds OperationPlan::to_doc to build an ExecutableDocument from the current JIT operation plan selection tree.

The conversion preserves:

• operation type
• selected fields and nested selections
• aliases via output_name
• field arguments with request-provided values
• custom directives
• variable-backed @skip / @include conditions
• flattened fragment fields as inline fragments

Issue Reference(s):
Algora bounty: https://algora.io/tailcallhq/bounties/cm1qcqviw0008jq03d10mtcd1

/claim https://algora.io/tailcallhq/bounties/cm1qcqviw0008jq03d10mtcd1

Build & Testing:

• I ran cargo test -p tailcall core::jit::model::test -- --nocapture successfully.
• I have run ./lint.sh --mode=fix to fix all linting issues raised by ./lint.sh --mode=check.

Checklist:

• I have added relevant unit & integration tests.
• I have updated the documentation accordingly.
• I have performed a self-review of my code.
• PR follows the naming convention of <type>(<optional scope>): <title>

[github-actions]

Action required: PR inactive for 5 days.
Status update or closure in 10 days.

[jaxassistant55]

Rebased this PR onto current main at 1afd8ffc and pushed updated commit 1a1cd260.

Validation rerun locally:

cargo test -p tailcall core::jit::model::test -- --nocapture

Result: 5 focused core::jit::model tests passed, 0 failed. GitHub PR title, release-draft, and CodeRabbit checks are green again; remaining blocker is maintainer review.

[github-actions]

Action required: PR inactive for 5 days.
Status update or closure in 10 days.

[jaxassistant55]

Rebased this PR onto current main at 184e8ffe and pushed updated commit ffd99a22.

Validation rerun locally:

cargo test -p tailcall core::jit::model::test -- --nocapture

Result: 5 focused core::jit::model tests passed, 0 failed. GitHub Check PR Title and update_release_draft checks are green again; remaining blocker is maintainer review.

[github-actions]

Action required: PR inactive for 5 days.
Status update or closure in 10 days.