PTHMINT-101: Add dev-only custom base URL support

[zulquer]

Allow passing a custom API base_url for local development while preventing custom URLs in non-dev builds. Client now accepts base_url and resolves it via _resolve_base_url (uses MSP_SDK_BUILD_PROFILE, MSP_SDK_ALLOW_CUSTOM_BASE_URL, MSP_SDK_CUSTOM_BASE_URL); _normalize_base_url validates/normalizes the URL. Sdk forwards base_url to Client. Added unit tests covering default test/live URLs, dev-only custom URL acceptance, env-provided URL, and blocking in release/when flag disabled. README updated with usage and guardrails.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.89%. Comparing base (ee15a54) to head (a19c304).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #52      +/-   ##
==========================================
+ Coverage   90.81%   90.89%   +0.08%     
==========================================
  Files         147      147              
  Lines        2579     2603      +24     
==========================================
+ Hits         2342     2366      +24     
  Misses        237      237              

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[Copilot]

Pull request overview

Adds a development-only mechanism to override the API base URL (via constructor parameter or env var) while enforcing guardrails to block custom URLs outside a “dev” profile.

Changes:

• Extend Client and Sdk to accept base_url and resolve it with environment-based guardrails.
• Add unit tests for default URL selection and dev/release allow/block behavior.
• Update README with guidance for enabling the dev-only override.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/multisafepay/client/client.py	Implements base URL resolution/normalization with dev-only guardrails.
src/multisafepay/sdk.py	Forwards base_url from Sdk to Client.
tests/multisafepay/unit/client/test_unit_client.py	Adds unit tests for default URLs and guardrail behavior at the Client level.
tests/multisafepay/unit/test_unit_sdk.py	Adds unit tests ensuring Sdk propagates URL behavior/guardrails.
README.md	Documents dev-only configuration and usage for custom base URL.

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