Validate and compile the user agent filter pattern once

[loevgaard]

Fixes #18

Stacked on #39.

Problem

FilterConfiguredUserAgentsSubscriber built '#' . implode('|', $userAgents) . '#' on every event and compared the result of preg_match() with === 1.

An invalid fragment, or one containing a #, makes preg_match() emit a warning and return false, which is not 1, so the filter silently stops filtering. foo#bar is the nastiest case: it closes the delimiter and turns bar into modifiers. Nothing failed, nothing was logged, bots simply started being tracked again.

Matching was also case sensitive, while the same crawler shows up in different casings.

Change

• The pattern is compiled once in the constructor. An invalid pattern now throws \InvalidArgumentException at container build time instead of failing open at runtime.
• Each fragment is validated in Configuration, so the error surfaces as a normal configuration error naming the offending fragment and reminding you to escape #.
• The i modifier is added, and the option is documented as case insensitive.
• The no-fragments case is decided once in the constructor rather than on every event.

Tests

Six unit tests (match, no match, case insensitive match, no user agent, no fragments configured, and both invalid-pattern constructor throws) plus two extension tests asserting the configuration is rejected.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.99%. Comparing base (985e748) to head (1b06004).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #40      +/-   ##
============================================
+ Coverage     62.40%   67.99%   +5.59%     
- Complexity      129      134       +5     
============================================
  Files            30       30              
  Lines           375      403      +28     
============================================
+ Hits            234      274      +40     
+ Misses          141      129      -12     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.