fix: reject invalid REST path params with descriptive validation errors

[SeqviriouM]

Invalid REST path parameters now return a 400 INVALID_PARAMS error instead of inserting GATEWAY_INVALID_PARAM_VALUE into the URL. The error identifies the first invalid parameter by its full path, and the upstream request is not sent.

Partial related issue #14

Summary by Sourcery

Reject invalid REST path parameters before request dispatch and return descriptive validation errors identifying the offending argument.

Bug Fixes:

• Reject invalid REST path parameters with a descriptive 400 INVALID_PARAMS validation error instead of inserting an invalid placeholder into the URL.
• Prevent upstream requests when path parameter validation fails and report the first invalid parameter using its full argument path.

Enhancements:

• Preserve existing schema-validation and unrelated path-function errors while supporting custom validation schemas.

Documentation:

• Document the new INVALID_PARAMS response details and upstream request behavior for invalid REST path parameters.

Tests:

• Add coverage for invalid nested, array, encoded, and unpaired Unicode path parameters, request suppression, custom schemas, and preserved errors.

[sourcery-ai]

Reviewer's Guide

REST path parameters are now validated during path construction and invalid values fail fast with a descriptive 400 INVALID_PARAMS error containing the first invalid parameter’s full path; no upstream request is sent, while schema validation and unrelated path errors retain their existing behavior.

File-Level Changes

Change	Details	Files
Reject invalid path parameters at the point where REST action paths are built, instead of substituting a sentinel URL value.	Added recursive path tracking for nested objects and arrays. Changed invalid values and URI-encoding failures to invoke a configurable invalid-parameter callback. Preserved opt-out behavior when path validation is disabled.	lib/utils/validate.ts lib/utils/validate.test.ts
Return a structured 400 validation error and prevent upstream requests when path validation fails.	Centralized error construction with status 400, code INVALID_PARAMS, and an Invalid path params title. Identified the first invalid parameter using its full argument path. Ended the context before propagating the rejected error, while preserving existing schema-validation behavior and unrelated path-function errors.	lib/components/rest.ts lib/index.test.ts
Document the new REST path-parameter validation contract.	Replaced the sentinel error behavior with the INVALID_PARAMS response contract. Documented the error title, parameter-path description, and upstream request prevention.	README.md

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[sourcery-ai]

Hey - I've reviewed your changes and they look great!

Sourcery assessment

Approved.

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