fix(scheduler): guard nil errors; drain request body; docs/tests

[hyp3rd]

• Add isNilError and avoid logging nil errors in logError (import reflect).
• Improve readBody to fully drain and tighten error handling.
• Align tests with defaultMaxRetries/defaultRetryCount; update expectations for attempts, intervals, and payload fields.
• Add tests: validation for unsupported method/invalid job, max-runs stop, and enforcing MaxBodyBytes for callback payloads.
• Update README with scheduler options, retry policy, schedule fields, and callback payload schema (job_id, scheduled_at, started_at, finished_at, attempts, success, status_code, error, response_body).

[Copilot]

Pull request overview

This PR improves error handling in the scheduler package by adding nil error guards, fixing a bug in the readBody function, and enhancing test coverage. The changes include adding comprehensive validation tests, aligning test expectations with defined constants, and updating documentation with detailed scheduler configuration options.

Changes:

• Fixed readBody to properly handle successful body draining without returning spurious errors
• Added isNilError helper to prevent logging typed nil errors
• Added validation tests for unsupported methods, invalid jobs, MaxRuns behavior, and callback payload body limits
• Replaced hardcoded test values with constants for better maintainability
• Enhanced README with scheduler options, retry policy details, and callback payload schema

Reviewed changes

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

File	Description
pkg/scheduler/scheduler.go	Added isNilError guard function using reflection, fixed readBody bug that incorrectly wrapped nil errors
tests/scheduler_test.go	Added validation tests for error cases, MaxRuns stopping behavior, and callback body size limits
tests/retrier_test.go	Replaced hardcoded retry values with defaultMaxRetries and defaultRetryCount constants
README.md	Added comprehensive documentation for scheduler options, schedule fields, retry policy, and callback payload structure

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