docs: add tag_keys field to request context examples

[detail-app]

Update request context JSON examples to include the tag_keys field,
which is now properly wired to the aws:TagKeys condition key.

Previously the code had a misleading comment that tag_keys was
automatically populated from request tags. The commit added explicit
wiring via the explicit_tag_keys() builder method, but the example
context files were not updated to show this field.

A developer reading the examples would not know that tag_keys can be
set explicitly in the request context JSON to control the aws:TagKeys
condition key value.

Introduced by commit 61b709b (@jplock, #23)

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