fix: update HTTP header constants to use proper capitalization

[HJLebbink]

• Update all X-Amz-* headers to use capital letters after hyphens (e.g., X-Amz-Date)
• Update standard HTTP headers to use proper case (e.g., If-Modified-Since)
• Update X-Minio headers to follow same capitalization pattern
• Aligns with AWS S3 API specification and HTTP standards

[Copilot]

Pull Request Overview

This PR updates HTTP header constants to use proper capitalization according to AWS S3 API specification and HTTP standards. The changes replace lowercase header names with proper title case format.

• Updates all X-Amz-* headers to use capital letters after hyphens (e.g., "x-amz-date" → "X-Amz-Date")
• Updates standard HTTP headers to use proper case (e.g., "if-modified-since" → "If-Modified-Since")
• Updates X-Minio headers to follow the same capitalization pattern

Reviewed Changes

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

File	Description
src/s3/header_constants.rs	Updates all HTTP header constants from lowercase to proper title case
tests/run-tests-windows.ps1	Adds environment variable fallback logic for MINIO_ROOT_USER/PASSWORD
CLAUDE.md	Adds comprehensive code style guide for the MinIO Rust SDK
AGENTS.md	Creates reference file pointing to CLAUDE.md

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