fix(limit-req): use parent resource key for consumer isolation

[janiussyafiq]

Description

Previously, when limit-req was configured at the consumer level, rate limits were applied per route instead of globally per consumer. This occurred because the key generation concatenated ctx.conf_type and ctx.conf_version, which included route-specific information. Now consumer-level rate limits are properly shared across all routes accessed by the same consumer by using the conf._meta.parent approach.

Changes:

• Used conf._meta.parent.resource_key for key generation
• Added consumer isolation test

Which issue(s) this PR fixes:

Fixes #12946

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)