Only accept a positive integer as the partner id

[loevgaard]

Fixes #51.

What changed

• PartnerIdParser::parse() (new, src/Parser) is the single place that turns a raw request value into a partner id: a positive integer, or null for anything else — empty, garbage, zero, negative, decimals, arrays. Uses filter_var(FILTER_VALIDATE_INT, min_range 1), so no casts.
• SetCookieSubscriber validates the affiliate query parameter through it and reads the raw value via query->all() instead of query->get(), so ?paid[]=x no longer makes InputBag::get() throw a BadRequestException (a 400 for the whole page). A mangled affiliate link now simply sets no cookie — and, importantly, no longer overwrites a legitimate earlier attribution with partner id 0.
• CookieHandler: has() is now only true when the cookie holds a valid partner id; get() asserts the same contract (interface docblocks updated). A tampered cookie is treated as absent.
• CreateConversionSubscriber: the partnerId <= 0 guard is gone — the cookie handler now guarantees a valid id, so it was dead code.

Tests

• New PartnerIdParserTest (data provider over valid/invalid inputs).
• SetCookieSubscriberTest and CookieHandlerTest gained data-provider cases for empty, garbage, zero, negative, decimal and array values, asserting no cookie is set and nothing throws.
• Infection: 100% MSI on the changed files.

Note: this tightens CookieHandlerInterface::has() semantics (a cookie with an invalid value now counts as absent) — deliberate, and in line with the major bump already on 3.x.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.90%. Comparing base (defc7a0) to head (4507884).

Additional details and impacted files

@@             Coverage Diff              @@
##                3.x      #59      +/-   ##
============================================
+ Coverage     84.79%   84.90%   +0.11%     
- Complexity      104      106       +2     
============================================
  Files            21       22       +1     
  Lines           546      550       +4     
============================================
+ Hits            463      467       +4     
  Misses           83       83              

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