Fix vulnerability ignore matching to be case-insensitive

[RohitYandigeri]

Summary

Vulnerability identifiers such as CVE and GHSA are case-insensitive, but ignore
rules were previously matched using exact string comparison. This caused ignore
rules to fail when casing differed between sources.

Root Cause

Ignore rule evaluation compared vulnerability IDs using strict string equality,
while vulnerability sources (e.g., Grype data, GHSA) may report identifiers in
mixed or lowercase formats.

Changes

• Make vulnerability ID matching case-insensitive when applying ignore rules
• Add a unit test validating that GHSA identifiers are ignored regardless of casing

Impact

• Fixes ignore rules not applying to GHSA vulnerabilities with lowercase IDs
• Backward compatible; does not affect existing ignore behavior

Notes

Local full test execution could not complete due to disk space constraints on the
development machine. The change is isolated to grype/match, and CI will validate
the full test suite.