fix(config)!: stop merging /etc and user client.conf across tiers

[mimi1vx]

Summary

client.conf discovery merged every existing file in a fixed two-path
list (/etc/openqa/client.conf then the user config dir), later keys
winning. That let a section in the user config combine with a section
in /etc/openqa across an administration boundary, and never
consulted /usr/etc/openqa or client.conf.d/*.conf drop-ins at all;
$OPENQA_CONFIG was also an exclusive override even when its
directory held no config.

config::default_paths now performs the same tiered lookup as current
openQA:

1. $OPENQA_CONFIG (only when set and non-empty)
2. $XDG_CONFIG_HOME/openqa (or ~/.config/openqa)
3. /etc/openqa, then /usr/etc/openqa

The first tier producing any file wins outright; later tiers are not
read at all. Within a tier, each directory contributes its
client.conf followed by its client.conf.d/*.conf drop-ins (sorted,
later wins), and a directory with only drop-ins does not stop the scan
of the tier's remaining directories.

Breaking change

Anyone relying on the old cross-tier merge — an override in
~/.config/openqa/client.conf combined with host sections that exist
only in /etc/openqa/client.conf — now loses the /etc sections
outright. $XDG_CONFIG_HOME as the tier-2 selector and treating a
malformed file as a hard error are kept as deliberate divergences from
upstream.

config::default_paths and ClientBuilder::config_paths keep their
existing signatures; an explicit config_paths list is still a plain
in-order merge with no tiering.

Testing

• cargo fmt --check
• cargo clippy --all-targets --locked -- -D warnings
• cargo test --locked
• cargo check --locked --all-targets
• cargo deny check