feat(ai-proxy): add defaults field for fallback model options

[sihyeonn]

Description

Add defaults field to ai-proxy and ai-proxy-multi plugins to provide fallback values for model options. Currently, options field always overrides user request values. This PR introduces a new defaults field that only applies when the user hasn't set a value in their request. Behavior:

• options: Always override user values (e.g., enforce specific model)
• defaults: Apply only when not present in user request (e.g., default max_tokens)

Example configuration:

{
"options": { "model": "gpt-4" },
"defaults": { "max_tokens": 512, "temperature": 0.7 }
}

User Request	Result
{"model": "gpt-3.5"}	{"model": "gpt-4", "max_tokens": 512, "temperature": 0.7}
{"max_tokens": 100}	{"model": "gpt-4", "max_tokens": 100, "temperature": 0.7}

Which issue(s) this PR fixes:

Fixes #

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)

[Baoyuantop]

1. Supplement Documentation - Update ai-proxy.md and ai-proxy.md, adding documentation for the defaults field
2. Supplement ai-proxy Plugin Tests - Add test cases to ai-proxy.openai-compatible.t or relevant test files
3. Link to Related Issues or Supplement Requirement Context - Specify the source of use cases for this feature

[sihyeonn]

Thanks for the review! I've addressed your feedback:

1. Added documentation for the defaults field in both ai-proxy.md and ai-proxy-multi.md (en/zh)
2. Added test cases to ai-proxy.openai-compatible.t
3. This feature addresses a common use case where admins want to set fallback values (like max_tokens, temperature) without overriding user preferences. Unlike options which always overrides, defaults only applies when users don't specify values in their requests.