feat: show request items separators in naked invocation usage

[TrueFurina]

Description

Running bare http (or making an argument error like http --pretty) prints only the one-line usage and the error. The full option reference is behind http --help (400+ lines in a pager), yet the thing users most often forget — per #428 — is the request item separators (==, =, :=, @, :), which appear in the usage line only as an opaque [REQUEST_ITEM ...] placeholder.

This PR adds a compact request-items cheatsheet to the naked-invocation/usage error output, right under the usage line:

usage:
    http [METHOD] URL [REQUEST_ITEM ...]
request items (common separators, run 'http --help' for all options):
    ==    Query params
    =     Data fields (string)
    :=    Data fields (raw JSON)
    @     File upload
    :     HTTP headers

error:
    the following arguments are required: URL

for more information:
    run 'http --help' or visit https://httpie.io/docs/cli

This follows the direction discussed in #428 (keeping http --help as the full reference while making the short output self-sufficient for the most common lookup).

Closes #428

Changelog

• httpie/output/ui/rich_help.py: add to_request_items_cheatsheet() rendering the common separators as plain text
• httpie/cli/argparser.py: print the cheatsheet under the usage line in print_usage()
• tests/test_cli_ui.py: extend NAKED_BASE_TEMPLATE to assert the new output

Testing

• python -m pytest tests/test_cli_ui.py — 4 passed (updated naked-invocation assertions)
• Manually verified http (bare), http --pretty, and http --pretty $invalid all render the cheatsheet correctly on stderr

Note: the full test suite was not run locally because many integration tests hit live network endpoints; CI will cover the rest.