tidy-up: minor code fixes and improvements

[vszakats]

• schannel: drop redundant parentheses.
• os400sys: drop redundant includes.
  Follow-up to ebc5212 tidy-up: drop redundant includes #22374
• pytest: replace () with [] to match rest of tests.
• libtests: constify some local pointers.
• libtests: drop redundant (long) casts.
• lib650: use CURL_CSTRLEN().
  Follow-up to 59dc2bb tidy-up: use CURL_CSTRLEN() macro on more static strings #22424

[Copilot]

Pull request overview

This PR applies a set of small “tidy-up” refactors across curl’s test suite and a couple of platform/TLS implementation files, focusing on removing redundancies and aligning code with established local patterns (e.g., CURL_CSTRLEN(), const-correctness, and consistent Python test style).

Changes:

• Replaces a sizeof(...)-N string-length idiom with CURL_CSTRLEN() in lib650 and removes redundant (long) casts in several libtests.
• Improves local const-correctness for struct curl_slist * iteration pointers in a few libtests.
• Removes redundant includes in the OS/400 source and drops redundant parentheses in schannel ALPN buffer pointer casts; adjusts a couple of pytest membership checks to use lists for consistency.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/libtest/lib650.c	Uses CURL_CSTRLEN() for a static string length expression (equivalent result to prior code).
tests/libtest/lib5004.c	Drops redundant (long) cast when passing CURLHTTPSIG_ED25519 (already a long constant).
tests/libtest/lib5000.c	Drops redundant (long) cast when passing CURLHTTPSIG_ED25519 (already a long constant).
tests/libtest/lib3207.c	Constifies a local curl_slist iteration pointer (no behavioral change).
tests/libtest/lib3102.c	Constifies a local curl_slist iteration pointer (no behavioral change).
tests/libtest/lib1582.c	Drops redundant (long) cast when passing CURLAUTH_NEGOTIATE (typecheck accepts unsigned/signed long).
tests/libtest/lib1549.c	Constifies a local curl_slist iteration pointer (no behavioral change).
tests/http/test_21_resolve.py	Uses list instead of tuple in an in check to match surrounding test style.
tests/http/test_02_download.py	Uses list instead of tuple in an in check to match surrounding test style.
projects/OS400/os400sys.c	Removes redundant sys/types.h and sys/socket.h includes (already pulled in via <curl/curl.h> on non-Windows).
lib/vtls/schannel.c	Removes redundant parentheses in two pointer-cast expressions (no behavioral change).

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