AP_OSD: fix camel-case buffer handling

[vlordier]

This draft isolates the AP_OSD camel-case buffer fixes from the larger hardening branch.

Summary:

• fix missing terminator / zero-length handling in AP_OSD parameter-name camel-case conversion
• add focused regression coverage for string termination, len==0, and len==1 edge cases

Why:

• this is a narrow string-handling fix that can be reviewed independently
• it avoids mixing OSD code with unrelated RC protocol, math, MAVLink, and tooling changes

Validation:

• branch was split cleanly from upstream/master
• local configure succeeded in an isolated worktree
• the branch includes focused regression test source for the camel-case buffer edge cases in this slice
• the target was not enumerated by the local worktree build as-is, so full test execution was not completed locally

This PR is intentionally draft while confirming the preferred test integration path.