Preserve tag-key case in context_key() (#14)

[jplock]

Fixes #14

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Note

Medium Risk
Changes IAM evaluation context key matching for tag-based conditions; correct for AWS semantics but may alter outcomes where policies depended on fully lowercased tag keys.

Overview
Fixes #14 by aligning how RequestContext stores and looks up keys from context_key(), context_key_multi(), and get_context_key().

A new normalize_context_key lowercases only the aws:PrincipalTag/, aws:ResourceTag/, and aws:RequestTag/ prefixes (prefix match is case-insensitive) and keeps the tag name after / unchanged, matching AWS case-sensitive tag keys. All other context keys are still fully lowercased.

Regression tests cover principal/resource/request tag keys, mixed-case prefixes, multi-value tags, non-tag keys, and round-trip via get_context_key.

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