fix(payload): build full jku/x5u URL in basic payload instead of bare domain

[hahwul]

Summary

The basic jku/x5u payload from the payload command emitted the raw
--jwk-attack value verbatim — a bare domain — as the header value:

jku = "evil.com"      # not a valid absolute URI, --jwk-protocol ignored

A JWKS/cert resolver that fetches this URL server-side would never reach the
attacker's endpoint (no scheme → not fetchable), so the primary and most
important URL payload was effectively non-functional. Meanwhile every other
jku/x5u variant already produced correct full URLs:

• the bypass variants use {protocol}://{trust}Z{domain}, …@…, …%0d%0a…
• the ssrf target emits complete URLs (http://169.254.169.254/…)

--jwk-protocol (default https, documented as http/https) was silently
ignored for the basic payload.

Fix

Prepend the configured scheme so the basic payload is a valid, fetchable URL
that honours --jwk-protocol, consistent with the bypass and SSRF variants. A
value that already carries a scheme is passed through verbatim so a
pre-schemed input never produces a double scheme (https://https://…).

Before / After

# jwt-hack payload <TOKEN> --target jku --jwk-attack evil.com --jwk-protocol http
before:  jku = "evil.com"
after:   jku = "http://evil.com"

Bypass variants are unchanged; --target ssrf and the scan command
(generate_jku_x5u_ssrf_payload) are unaffected.

Tests

• Updated test_generate_url_payload_jku_basic and
  test_generate_url_payload_x5u_with_trust to expect the full URL.
• Added test_generate_url_payload_basic_respects_protocol covering the
  https default, --jwk-protocol=http, and the pre-schemed pass-through
  (no double scheme).
• cargo test (558 tests), cargo fmt --check, cargo clippy --all-targets
  all clean.

Scope / constraints

• Only src/payload/mod.rs touched (within assigned scope).
• JSON serialization contract unchanged (only a payload value is fixed;
  structure/keys/format untouched). Token lines remain structurally intact.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!