feat(api): add configurable base URL for fetcher and mutator

[marklearst]

Add optional baseUrl parameter to FetcherOptions and MutatorOptions interfaces, allowing users to override the default Figma API base URL.

Why:

• Enables testing against local mocks without patching constants
• Supports proxy scenarios for enterprise environments
• Allows integration with custom API gateways
• Prepares for future self-hosted or alternative endpoints

Changes:

• src/api/fetcher.ts: Added baseUrl option with default 'https://api.figma.com'
• src/api/mutator.ts: Added baseUrl option with default 'https://api.figma.com'
• tests/api/fetcher.test.ts: Added 3 tests for baseUrl override behavior
• tests/api/mutator.test.ts: Added 3 tests for baseUrl override behavior

Usage:
const data = await fetcher('/v1/files/abc/variables/local', token, { baseUrl: 'http://localhost:3000' })

Note: When URL is already absolute (starts with http:// or https://), the baseUrl option is ignored.

Refs: Codex Audit Item #10

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.47%. Comparing base (78f89d7) to head (9537ec2).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #11      +/-   ##
==========================================
+ Coverage   90.36%   90.47%   +0.10%     
==========================================
  Files          34       34              
  Lines         893      903      +10     
  Branches      245      246       +1     
==========================================
+ Hits          807      817      +10     
  Misses         85       85              
  Partials        1        1              

Flag	Coverage Δ
unittests	90.47% <100.00%> (+0.10%)	⬆️

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[Copilot]

Pull request overview

This PR adds support for configurable base URLs in the fetcher and mutator functions to enable testing against local mocks, proxy scenarios for enterprise environments, and integration with custom API gateways.

• Added optional baseUrl parameter to FetcherOptions and MutatorOptions interfaces with default value of https://api.figma.com
• Updated URL construction logic to use the configurable base URL instead of the hardcoded constant
• Added comprehensive test coverage for base URL override scenarios

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/api/fetcher.ts	Added baseUrl option to FetcherOptions interface and updated URL construction to use it
src/api/mutator.ts	Added baseUrl option to MutatorOptions interface and updated URL construction to use it
tests/api/fetcher.test.ts	Added tests covering custom baseUrl, default baseUrl, and absolute URL scenarios
tests/api/mutator.test.ts	Added tests covering custom baseUrl, default baseUrl, and absolute URL scenarios

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