Redact sensitive parts of remote config URLs in errors and improve fetch error messages

[winhowes]

Motivation

• Prevent leaking credentials, query parameters, or fragments from config URLs into error messages when fetching remote configs.
• Make remote fetch error messages clearer and more descriptive for debugging.

Description

• Use redactConfigSource to produce a redacted src and reference it in openSource error messages instead of the raw path and enhance messages to say invalid URL, request failed, or remote fetch ... failed where appropriate.
• Introduce redactSourceInError to replace occurrences of the raw source in underlying error text with the redacted source.
• Change redactConfigSource parsing behavior to return the original input when URL parsing fails and keep a separate check for empty Scheme and Host.
• Update and add unit tests to reflect the new messages and redaction behavior, including TestOpenSourceHTTPDialErrorRedactsSecrets and an additional TestRedactConfigSource case for invalid URLs.

Testing

• Ran package unit tests with go test ./... covering openSource and redactConfigSource behavior.
• All modified and added tests (including TestOpenSourceHTTPStatus, TestOpenSourceHTTPRequestError, TestOpenSourceHTTPDialErrorRedactsSecrets, and TestRedactConfigSource) passed successfully.

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Codex Task

[winhowes]

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[winhowes]

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Codex Review: Didn't find any major issues. 🚀

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