Add ngx_js_form_fuzzer for HTTP form body parsing

[kuranikaran]

njs 0.9.9 added nginx/ngx_js_form.c, an ~860-line multipart and urlencoded form body parser exposed through the r.readRequestForm() API. The existing njs_process_script_fuzzer only covers the JS engine script path and doesn't reach this parser.
Since this code handles untrusted HTTP request bodies (Content-Type parsing, boundary matching, percent-decoding, Content-Disposition header extraction, quoted-string unescaping), it's a high-value fuzzing target.
This PR adds a dedicated fuzz harness that drives ngx_js_parse_form() directly with both urlencoded and multipart inputs through minimal nginx core stubs (pool, array, string helpers), avoiding the need for a full nginx build.
What's included:

fuzz/ngx_js_form_fuzzer.cc — harness source
fuzz/stubs/ — minimal nginx type/function stubs (ngx_pool, ngx_array, ngx_str)
fuzz/seed_corpus/ — 8 seed inputs covering normal, truncated, oversized-boundary, quoted-filename, and empty-body paths
fuzz/ngx_js_form_fuzzer.dict — fuzzer dictionary

This is being coordinated with a corresponding oss-fuzz integration PR to wire up the new fuzzer in Google's continuous fuzzing infrastructure.
Note: This PR intentionally does not modify auto/make or the build system. The oss-fuzz integration (google/oss-fuzz#15533) injects the build target dynamically. If you'd prefer the target wired into auto/make natively, happy to add that.

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[kuranikaran]

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[kuranikaran]

Hi @xeioex - this adds a fuzzer for the new form body parser in ngx_js_form.c. Would appreciate your review when you get a chance. The corresponding oss-fuzz integration is at google/oss-fuzz#15533.