curl_formdata: fix to pass long where missing, document CURLFORM_NAMELENGTH

[vszakats]

• lib650: pass long to CURLFORM_NAMELENGTH in test.
  Spotted by Copilot.
  libtests: prefer sizeof() over strlen(), fix potential OOB read in 1591 #22011 (comment)
  Follow-up to 3620e56

• lib650: drop an interim variable, and interim casts.
  Follow-up to 60776a0 Enable and fix more GCC warnings #2747

• curl_formdata.md: document CURLFORM_NAMELENGTH on man page.

• curl_formdata.md: pass long to CURLFORM_BUFFERLENGTH on man page.

• formdata: pass long to CURLFORM_CONTENTSLENGTH in comment.

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https://github.com/curl/curl/pull/22017/files?w=1

There is also this in lib650, I couldn't trace back in source
if size_t is correct or should be long (I'd guess it's the
latter):

  formarray[1].option = CURLFORM_CONTENTSLENGTH;
  formarray[1].value = (char *)(size_t)contentlength;

[Copilot]

Pull request overview

This PR tightens correctness and consistency around the deprecated curl_formadd() form API by ensuring options that expect long are documented/passed as long, and by cleaning up a few documentation/examples.

Changes:

• Update docs/examples to explicitly pass long to CURLFORM_BUFFERLENGTH and document CURLFORM_NAMELENGTH.
• Fix tests/libtest/lib650.c to pass a long for CURLFORM_NAMELENGTH (and simplify an interim variable).
• Minor whitespace/style cleanups in docs and comments.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 1 comment.

File	Description
tests/libtest/lib650.c	Fixes CURLFORM_NAMELENGTH argument type to long in the test and simplifies length handling.
lib/formdata.c	Adjusts an internal comment example to use 10L and cleans whitespace.
docs/libcurl/opts/CURLOPT_COPYPOSTFIELDS.md	Formatting/whitespace tweak in an example.
docs/libcurl/curl_formadd.md	Adds CURLFORM_NAMELENGTH section and fixes CURLFORM_BUFFERLENGTH example to cast to long.

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[Copilot]

Pull request overview

Copilot reviewed 3 out of 4 changed files in this pull request and generated no new comments.