test: add cross-flow isolation test for HTTP parse errors

[Zious11]

Context

Discovered during PR review of #17 (HTTP parse error counter). The test coverage reviewer noted that all HTTP analyzer tests use a single test_flow_key(). No test verifies that parse errors on one flow don't affect parsing of a different flow.

Proposed Test

Create two flow keys; send malformed data on flow A, then valid data on flow B; assert flow B parses correctly and total parse errors is 1 (only from flow A).

This tests pre-existing flow isolation behavior in HttpAnalyzer.flows HashMap, not specific to the parse error counter — hence deferred from #17.

Files

• tests/http_analyzer_tests.rs