refactor(errors): separate ApiError from RequestError

[JuroUhlar]

• Add ApiError extends RequestError, thrown for structured Server API error responses; it carries the strongly typed errorCode (ErrorCode) and responseBody: ErrorResponse.
• TooManyRequestsError now extends ApiError; the client throws ApiError for structured responses.
• RequestError becomes the base for request-level errors, thrown directly for non–Server-API responses (e.g. an intermediate proxy). It no longer declares errorCode.
• Readme error-handling section updated to show ApiError/ErrorCode narrowing.

Stacked on #251

Base is the strongly-typed-errorCode branch (#251). Review/merge #251 first; this PR's diff is only the error separation.

Runtime change: proxy-path errorCode

On the non–Server-API path, errorCode previously reflected response.statusText; it is now absent (undefined). The compiler can't catch this for untyped consumers, so it's a runtime change, not just a type change — flagging against the repo's "runtime API follows semver strictly" note. Shipped as minor; happy to bump to major if preferred.

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🦋 Changeset detected

Latest commit: 8baa87a

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[github-actions]

🚀 Following releases will be created using changesets from this PR:

@fingerprint/node-sdk@7.5.0

Minor Changes

• Separate Server API errors from other request errors.

  • Added ApiError (extends RequestError), thrown when the Server API returns a structured error response. It carries the strongly typed errorCode (ErrorCode) and responseBody: ErrorResponse.
  • TooManyRequestsError now extends ApiError.
  • RequestError is now the base class for request-level errors and is thrown directly for non–Server-API responses (for example, errors returned by an intermediate proxy). It no longer declares errorCode.

  Migration: ApiError extends RequestError, so error instanceof RequestError checks keep working. To read the error code, narrow to ApiError (if (error instanceof ApiError) { error.errorCode }).

  Runtime note: on the non–Server-API (proxy) error path, errorCode previously reflected response.statusText; it is now absent (undefined). This is a runtime change for untyped consumers that read errorCode on such errors, not merely a type change. (8baa87a)

• Strongly type the RequestError.errorCode field.

  errorCode is now typed as the ErrorCode union (the set of error codes returned by the Fingerprint Server API) instead of a free-form string. The new ErrorCode type is also exported. This is a type-only change; runtime behavior is unchanged. (a83ad4b)

[github-actions]

Coverage report

St.❔	Category	Percentage	Covered / Total
🟢	Statements	100%	346/346
🟢	Branches	100%	115/115
🟢	Functions	100%	36/36
🟢	Lines	100%	346/346

Test suite run success

78 tests passing in 22 suites.

Report generated by 🧪jest coverage report action from 8baa87a

Show full coverage report

St	File	% Stmts	% Branch	% Funcs	% Lines	Uncovered Line #s
🟢	All files	100	100	100	100
🟢	src	100	100	100	100
🔴	...edApiTypes.ts	0	0	0	0
🟢	index.ts	100	100	100	100
🟢	sealedResults.ts	100	100	100	100
🟢	...rApiClient.ts	100	100	100	100
🟢	types.ts	100	100	100	100
🟢	urlUtils.ts	100	100	100	100
🟢	webhook.ts	100	100	100	100
🟢	src/errors	100	100	100	100
🟢	apiErrors.ts	100	100	100	100
🟢	...orResponse.ts	100	100	100	100
🟢	toError.ts	100	100	100	100
🟢	unsealError.ts	100	100	100	100