test: Implement integration tests

[tvpeter]

Description

This PR builds on #278 and introduces integration tests for bdk-cli. It replaces manual std::process::Command boilerplate with the assert_cmd library, allowing us to perform black-box testing against the compiled binary.

Features covered so far include:

• key: generate, derive, and restore
• wallets: list wallets
• wallet config: save config, read config.
• descriptor: generate descriptor
• compile: policy compiler
• offline wallet operations: new_address, unused_address, balance, unspent, transactions, policies, public_descriptor, create_tx, combine_psbt

Notes to the reviewers

Changelog notice

• Introduces the BdkCli helper struct to inject context state into base commands.
• Dropped verbose flag from wallets as it was applicable to only Pbst

Checklists

All Submissions:

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

[codecov]

Codecov Report

❌ Patch coverage is 82.50000% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.89%. Comparing base (63f769a) to head (6000dd5).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/main.rs	0.00%	8 Missing ⚠️
src/handlers/repl.rs	33.33%	4 Missing ⚠️
src/config.rs	96.66%	1 Missing ⚠️
src/utils/runtime.rs	93.33%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #289       +/-   ##
===========================================
+ Coverage   24.79%   56.89%   +32.10%     
===========================================
  Files          22       22               
  Lines        3678     3661       -17     
===========================================
+ Hits          912     2083     +1171     
+ Misses       2766     1578     -1188     

Flag	Coverage Δ
rust	56.89% <82.50%> (+32.10%)	⬆️

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