Lock config/cache files during read-modify-write

[elhoim]

Finding 18 (Medium) — bin/cli.py:706

Problem

load_config/save_config and load_cache/save_cache have no locking around the read-modify-write. Two concurrent --configure-module runs each load, mutate and overwrite the whole file; the last writer silently discards the other's credential.

Fix

Finding #18 (config-file-locking, medium): load_config/save_config and load_cache/save_cache had no locking around their read-modify-write cycle, so two concurrent --configure-module runs (or two concurrent queries populating the cache) could each load, mutate and overwrite the whole file — the last writer silently discarding the other's saved credential/cache entry. Added a locked_file() context manager (bin/cli.py) that takes an exclusive flock on a sibling <path>.lock file, and used it to serialize (1) the configure_module load-mutate-save sequence around config_path, and (2) the cache save at the end of main, which now re-reads the cache file under the lock and merges in this run's new entries before writing, so a concurrent run's cache writes made in the meantime aren't clobbered.

Verification

Reproduced against the unmodified code at 9b8c605, then re-checked after the change.

Before

Two subprocess CLI-style workers (worker2.py) both call load_config -> mutate -> save_config on the same /tmp/race_config.json, worker A sleeping 1.0s between load and save, worker B sleeping only 0.1s and starting 0.3s later:

[moduleA] loaded at 0.000: {'modules': {}}
[moduleB] loaded at 0.000: {'modules': {}}
[moduleB] saved at 0.109: {'modules': {'moduleB': {'apikey': 'secretB'}}}
[moduleA] saved at 1.017: {'modules': {'moduleA': {'apikey': 'secretA'}}}

Final /tmp/race_config.json:
{
  "modules": { "moduleA": { "apikey": "secretA" } }
}

moduleB's saved credential is silently lost — worker A's stale in-memory copy (loaded before B's write) overwrote the whole file.

After

Same two workers, now acquiring locked_file(CONFIG) around the whole load-mutate-save sequence:

[moduleB] loaded at 0.027: {'modules': {}}
[moduleB] saved at 0.131: {'modules': {'moduleB': {'apikey': 'secretB'}}}
[moduleA] loaded at 0.000: {'modules': {'moduleB': {'apikey': 'secretB'}}}   (blocked on lock until B released it, then re-read)
[moduleA] saved at 1.017: {'modules': {'moduleB': {'apikey': 'secretB'}, 'moduleA': {'apikey': 'secretA'}}}

Final /tmp/race_config.json:
{
  "modules": {
    "moduleA": { "apikey": "secretA" },
    "moduleB": { "apikey": "secretB" }
  }
}

Both credentials survive. Also verified: `python bin/cli.py --help` exits 0, and the module imports cleanly via importlib (locked_file present on the module).

python bin/cli.py --help exits 0 and the module still imports cleanly. Verification was performed offline against the pure functions — no running misp-modules instance is required.

Branched from 9b8c605. This PR addresses only this finding; the other findings from the same review are in separate PRs, so they will need rebasing against each other as they merge.

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