fix: resolve Promise.all concurrent query collision with identical SQ…

[imsanupm]

Description

Fixes #1953

When two async functions with identical SQL strings were executed
concurrently via Promise.all, no results were returned and no error
was thrown.

Root Cause

Two separate issues were working together to cause this bug:

1. cleanupCache mutating shared cached object — The compiled
   statement stored in sqlCache is shared across concurrent executions.
   cleanupCache was clearing statement.query.data = [] on this shared
   object while another concurrent call was still reading it, silently
   wiping the results.

2. Shared params reference — Concurrent executions via Promise.all
   were sharing the same params reference, allowing one call to overwrite
   another's parameters (f1/f2).

Why it was hard to notice

• Running functions individually worked fine
• Running with different SQL strings (different LIMIT values) worked
  fine because they got different cache keys
• No error was thrown — results were just silently empty
• Only triggered by concurrent identical SQL strings via Promise.all

Changes

• cleanupCache() — Removed mutation of statement.query.data on the
  shared cached statement object
• alasql.dexec() — Clone params before passing to cached statement
  execution to isolate each concurrent call's parameters

How to reproduce (before fix)

async function fun1() {
    return await alasql.promise(`select * from xlsx(?)`, ["D:\\a1.xlsx"]);
}
async function fun2() {
    return await alasql.promise(`select * from xlsx(?)`, ["D:\\a2.xlsx"]);
}

// Returns empty, no error thrown
const results = await Promise.all([fun1(), fun2()]);

Testing

• Identical SQL strings in Promise.all now return correct results
• Different SQL strings still work as before
• Sequential (non-concurrent) queries unaffected
• Queries without cache (alasql.options.cache = false) unaffected