check and merge config from toml and env

[asmogo]

Description

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Adds upfront, fail-fast validation of cdk-mintd settings after TOML + env-var merge, so misconfiguration produces a clear error pointing at both the offending TOML key and env var instead of a silent default or a confusing downstream crash.

Notes to the reviewers

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• New validate_settings() covers listen address, signing source (mnemonic / seed length / signatory URL), per-backend lightning config, database, auth, management-RPC, and Prometheus.
• #[serde(default)] added across config structs so partial TOML files merge cleanly with defaults and env vars.
• Settings::new split into try_new (Result) + new (panics on any error) — removes the old silent-fallback-to-defaults footgun.
• Replaces an .expect() panic in Prometheus server startup with a propagated error.
• Adds a shared test-utils module (env-mutex + unique temp dirs) and a full set of negative tests for every validator branch.

Suggested CHANGELOG Updates

CHANGED

ADDED

REMOVED

FIXED

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Checklist

• I followed the code style guidelines
• I ran just quick-check before committing
• If the Wallet API was modified (added/removed/changed), I have reflected those changes in the FFI bindings (crates/cdk-ffi)

[codecov]

Codecov Report

❌ Patch coverage is 85.60606% with 76 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.44%. Comparing base (8ce06b1) to head (d3a4185).

Files with missing lines	Patch %	Lines
crates/cdk-mintd/src/lib.rs	85.40%	75 Missing ⚠️
crates/cdk-mintd/src/config.rs	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1962      +/-   ##
==========================================
+ Coverage   65.16%   65.44%   +0.27%     
==========================================
  Files         330      330              
  Lines       56742    57254     +512     
==========================================
+ Hits        36976    37469     +493     
- Misses      19766    19785      +19     

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