fix(ai-proxy): abstract a base for ai-proxy

[shreemaan-abhishek]

Description

In the previous ai-proxy-multi feat PR: #11986, the ai-proxy-multi module would overwrite the ai-proxy module functions.

Due to this, the ai-proxy plugin would execute ai-proxy-multi plugin's logic. This issue was found during some manual tests. The CI could not detect because it could not run properly as test::nginx could not initialise an nginx process as it could not find a certificate specified in the following test:

apisix/t/plugin/ai-proxy-multi2.t

Lines 291 to 296 in cc7441f

	=== TEST 6: send request
	--- custom_trusted_cert: /etc/ssl/cert.pem
	--- request
	POST /anything
	{ "messages": [ { "role": "system", "content": "You are a mathematician" }, { "role": "user", "content": "What is 1+1?"} ] }
	--- error_code: 401

The CI failure log: https://github.com/apache/apisix/actions/runs/13489934366/job/37686455127#step:19:190

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)

[membphis]

CI failed