Fix Query.Clone() shallow copy of Includes and Variables

[Copilot]

Query.Clone() and XQuery.Clone() assigned Includes and Variables by reference, so mutations on the clone leaked back to the original.

var query = new Query("users")
    .Include("posts", new Query("posts"), "user_id", "id");

var clone = query.Clone();
clone.Includes[0].Name = "modified";   // mutates original!
clone.Include("comments", ...);        // adds to original's list!
clone.Define("x", 1);                  // adds to original's Variables!

Changes

• Include: Added Clone() method that deep-copies all fields, including recursively cloning the embedded Query.
• Query.Clone(): Replace reference assignments with Includes.Select(i => i.Clone()).ToList() and new Dictionary<string, object>(Variables).
• XQuery.Clone(): Same fixes applied for consistency.
• Tests: Added four regression tests covering independent list mutation, independent object property mutation, independent variable dictionary mutation, and full property preservation.

Original prompt

Create a pull request that fixes the bug described in PR #748 (“Fix Clone() in Query.”) for repository sqlkata/querybuilder.

Context:

• The user provided PR metadata: Fix Clone() in Query. #748
• Goal: “check this issue” (the bug around Query.Clone()), and implement a PR to fix it.

Requirements:

1. Inspect the current implementation of Query.Clone() (and any related cloning/copying methods) and determine what is wrong.
2. Implement a correct fix for Clone() so that cloning a Query produces an independent copy without shared mutable state (e.g., lists, dictionaries, clauses, bindings, variables, includes, components, etc.).
3. Add or update unit tests to reproduce the bug and ensure it remains fixed.
4. Ensure all existing tests pass.
5. Keep the fix minimal and consistent with the project’s patterns.

Notes:

• Base branch: main.
• Use clear commit messages and PR title describing the fix.
• If there are multiple query types (e.g., Query, XQuery, SqlServerQuery, etc.), ensure cloning behavior is correct across them or document why only Query needs changes.

The following is the prior conversation context from the user's chat exploration (may be truncated):

User: Can you check this issue, and implement a PR to fix it?

Assistant: I’ve queued up the changes to create a fix PR for Query.Clone(), but I’m currently paused waiting for your approval/confirmation in the tool dialog—once you accept, I’ll proceed with implementing the fix and adding regression tests.

This pull request was created from Copilot chat.