fix(datadir): enforce strict permissions on datadir and config.toml

[tvpeter]

Description

wallet config persisted descriptors including xprv/tprv to config.toml via fs::write, and created the datadir / wallet dirs with umask defaults. Any other local user or process that could read the home tree could recover the account-level key and sweep the wallet. The existing warning only mentioned plaintext storage, not world-readability.
This PR creates new datadirs and wallet/payjoin subdirs with DirBuilder mode 0700 on Unix (create_restricted_dir), writes config.toml with OpenOptions mode 0600. On load/save, it hardens an existing config.toml (and the default ~/.bdk-bitcoin on startup) when group/other bits are set, and prints a warning when it does so.
Also, adds unit and integration tests that a private-descriptor config is 0600 and that a stale 0644 file is repaired on load and save.

Behaviour is unchanged for windows. wallet.sqlite / payjoin DB files may still be created 0644 by the DB libraries, they sit under a 0700 parent.

Fixes #331

Notes to the reviewers

• limit_access only acts when mode & 0o077 != 0, so an already owner-only file (0400) is not widened to 0600.
• Payjoin’s create_dir_all was the same umask defaults and is included so a session dir is not left 0755 beside the wallet.

Changelog notice

• Fixed the data directory and config.toml permission being world-readable (0755/0644) to 0700/0600 on Unix.

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[codecov]

Codecov Report

❌ Patch coverage is 97.67442% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.06%. Comparing base (682e414) to head (b2b99b4).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
src/utils/common.rs	98.00%	2 Missing ⚠️
src/config.rs	96.42%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #334      +/-   ##
==========================================
+ Coverage   57.78%   59.06%   +1.27%     
==========================================
  Files          22       22              
  Lines        3733     3857     +124     
==========================================
+ Hits         2157     2278     +121     
- Misses       1576     1579       +3     

Flag	Coverage Δ
rust	59.06% <97.67%> (+1.27%)	⬆️

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[notmandatory]

Also wondering why "Custom --datadir is intentionally not auto-hardened. Passing --datadir ~/.bdk-bitcoin is Some(_), so that path is also not repaired as default." Is there some reason I'm missing not to also harden a non-default datadir path?

[tvpeter]

Also wondering why "Custom --datadir is intentionally not auto-hardened. Passing --datadir ~/.bdk-bitcoin is Some(_), so that path is also not repaired as default." Is there some reason I'm missing not to also harden a non-default datadir path?

I thought that a custom directory might likely be existing before and may contain other contents, and hardening it might prevent other users from accessing it. But looking at it again, that is not a sound decision. So I will update it.

[tvpeter]

Also wondering why "Custom --datadir is intentionally not auto-hardened. Passing --datadir ~/.bdk-bitcoin is Some(_), so that path is also not repaired as default." Is there some reason I'm missing not to also harden a non-default datadir path?

The PR has now been updated and the exception for passed custom data directories has been removed. The other minor changes have also been effected.
Thank you.

[notmandatory]

ACK 0808206

Thanks for the fixes and cleanup with the DIR_MODE and FILE_MODE consts.

[tvpeter]

Force-pushed a rebase on master.