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

[DanielWu-star]

Previously, when limit-conn was configured at the consumer level, connection 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.

This commit fixes the issue by using conf._meta.parent.resource_key for key generation, consistent with the same fix already applied to limit-req (#13019) and limit-count plugins. Now consumer-level connection limits are properly shared across all routes accessed by the same consumer.

Changes:

• Added gen_limit_key() function using parent.resource_key approach
• Replaced key = key .. ctx.conf_type .. ctx.conf_version with gen_limit_key(conf, ctx, key) in run_limit_conn()
• Updated test cases to match new key format
• Added test cases (TEST 35-42) verifying consumer isolation across multiple routes

Description

Which issue(s) this PR fixes:

Fixes #13584

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)