Address clippy issues (take 2)

[imobachgs]

Problem

There are many complains from Clippy.

Solution

• Apply automatic suggestions.
• Update minimal Rust version to 1.88 (version of the rust package in SLES).
• Ask Gemini to fix the rest:
  • Boxed large enum variants in agama-software and agama-utils to reduce memory footprint and satisfy large-enum-variant and result-large-err lints.
  • Updated function signatures to use &Path instead of &PathBuf and &[T] instead of &Vec to avoid unnecessary allocations (ptr-arg lint).
  • Replaced manual boolean comparisons with idiomatic assert! and matches! macros.
  • Removed redundant imports, unused variables, and dead code across multiple crates.
  • Fixed documentation formatting and improved code readability by collapsing nested if let statements.
  • Simplified pointer casts and updated struct initializations to be more concise.

[jreidinger]

I would probably fix that differently then silencing warning

[imobachgs]

I agree but I think it does not matter because this code should be removed (is the old iSCSI HTTP client).

[jreidinger]

I do not know what is reason here for manual cleaning?

[imobachgs]

From Gemini: By calling wait(), the parent process acknowledges the child's termination, allowing the operating system to fully remove the process from the table and release its PID. In this specific Drop implementation, it ensures that every D-Bus daemon started for a test is cleanly and completely removed when the test finishes or fails.