[BUG] Cannot parse ISO format when calcite enable

[xinyual]

What is the bug?
When we enable calcite, ppl cannot parse ISO format.

How can one reproduce the bug?
Steps to reproduce the behavior:

PUT /demo
{
    "mappings": {
        "properties": {
            "time": {
                  "type": "date"
                }
        }
    }
}
PUT  /_cluster/settings/
{
  "transient" : {
    "plugins.calcite.enabled": true,
    "plugins.calcite.pushdown.enabled": true
  }
}

Then run PPL

POST /_plugins/_ppl/
{
    "query": "source = demo | where time>'2025-08-25T00:00:00Z'"
}

Get error
timestamp:2025-08-25T00:00:00Z in unsupported format, please use 'yyyy-MM-dd HH:mm:ss[.SSSSSSSSS]'

What is the expected behavior?
A clear and concise description of what you expected to happen.

What is your host/environment?

• OS: [e.g. iOS]
• Version [e.g. 22]
• Plugins

Do you have any screenshots?
If applicable, add screenshots to help explain your problem.

Do you have any additional context?
Add any other context about the problem.