fix: use tempfile in test_serde_config_args for CI reliability

[sanity]

Problem

The config::tests::test_serde_config_args test was failing intermittently on our self-hosted CI runners with:

thread 'config::tests::test_serde_config_args' panicked at crates/core/src/config/mod.rs:1264:38:
called `Result::unwrap()` on an `Err` value: Permission denied (os error 13)

This blocked 5 of 7 Dependabot PRs from passing CI, even though the test failure was unrelated to the dependency updates.

Root Cause

The test used id: Some(unique_id) which triggers ConfigPathsArgs::default_dirs() to create directories under /tmp/freenet-{id}. Self-hosted runners can have restrictive permissions on /tmp directories that prevent this.

Solution

Use tempfile::tempdir() to create a guaranteed-writable temporary directory, then explicitly set config_paths.config_dir and config_paths.data_dir to that path. This bypasses the /tmp code path entirely.

Testing

• Verified locally that the test passes
• The fix is minimal and low-risk since it only affects test code

Impact

Once merged, the 5 blocked Dependabot PRs should pass CI:

• chore(deps): bump freenet-test-network from 0.1.4 to 0.1.8 #2189 (freenet-test-network)
• chore(deps): bump test-log from 0.2.18 to 0.2.19 #2188 (test-log)
• chore(deps): bump tracing from 0.1.41 to 0.1.43 #2186 (tracing)
• chore(deps): bump bytesize from 2.3.0 to 2.3.1 #2185 (bytesize)
• chore(deps): bump tracing-subscriber from 0.3.18 to 0.3.22 in /tests/test-contract-1 #2179 (tracing-subscriber in test-contract-1)

[AI-assisted - Claude]