Backport-4.6: disallow setting of readonly/internal parameters via mavftp

[peterbarker]

Original PR is here: #29988

[Copilot]

Pull Request Overview

This PR backports changes to disallow setting of readonly/internal parameters via the MAVFTP interface. The changes centralize validation logic through a new method and update related components and tests accordingly.

• Updated the GCS_MAVLink parameter handler to use allow_set_via_mavlink().
• Added the allow_set_via_mavlink() method to AP_Param classes to control access via MAVLink.
• Modified the filesystem parameter upload and added an automated test to ensure readonly parameters cannot be overwritten.

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
libraries/GCS_MAVLink/GCS_Param.cpp	Replaced legacy internal flag check with allow_set_via_mavlink() to enforce set rules.
libraries/AP_Param/AP_Param.h	Declared the new allow_set_via_mavlink() method.
libraries/AP_Param/AP_Param.cpp	Defined allow_set_via_mavlink() to combine readonly and internal checks.
libraries/AP_Filesystem/AP_Filesystem_Param.cpp	Applied allow_set_via_mavlink() check in filesystem parameter uploads.
Tools/autotest/arducopter.py	Added test ReadOnlyDefaults to verify readonly parameter enforcement via MAVFTP.

Comments suppressed due to low confidence (1)

Tools/autotest/arducopter.py:12189

• [nitpick] Consider renaming ReadOnlyDefaults to follow Python's snake_case naming convention (e.g. test_read_only_defaults) to maintain consistency with the project style.

def ReadOnlyDefaults(self):