Fix datetime placeholders when piped from other nodes

[altoiddealer]

Resolves #9503

Currently: %date:FORMAT% placeholders in fields like filename_prefix are only expanded when entered directly in the node UI. This works because the frontend handles the substitution before passing the value to the backend.

However, if the same string is piped in from another node (e.g. String, Combine Text, etc.), the placeholder is passed to the backend unchanged. Since the backend’s compute_vars function only supports a fixed set of tokens (%year%, %month%, %day%, etc.), %date:...% values are not resolved. This leads to incorrect or invalid filenames such as:

yyyy-MM-ddhhmmss_00001.png

or, on Windows, runtime errors like:

OSError: [Errno 22] Invalid argument: '...\\%date:yyyy-MM-dd%_%date:hhmmss%__00001.png'

---

Fix

This PR adds backend support for %date:FORMAT%.

• Introduces a regex-based handler for %date:...%

• Maps common Java/ISO-style patterns (yyyy, MM, dd, hh, mm, ss) to Python’s strftime equivalents

• Ensures consistent substitution regardless of the source of the format string

Result

With this change, both direct inputs and piped inputs produce correctly formatted filenames. For example:

Input:

WAN/%date:yyyy-MM-dd%%date:hhmmss%

Output:

WAN/2025-08-23_143752_00001.png

This makes datetime substitution consistent across all nodes and prevents invalid filenames.

[silveroxides]

This is already included in #8380