fix(plugin): align unavailable plugin handling

[shreemaan-abhishek]

Description

• Keep authoritative HTTP and stream validation strict, so unavailable plugin names are rejected before persistence.
• Keep data-plane etcd loading tolerant when an existing configuration references a plugin unavailable on that node.
• Warn when an unavailable plugin is skipped; remain silent when _meta.disable: true makes the skip explicit.
• Pass the etcd key consistently to checkers and treat only literal true as strict stream validation.

Which issue(s) this PR fixes:

N/A

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)