Support referencing agent variables in agent tool form parameters

[obadakhalili]

Summary

depends on #13165.

this PR proposes a change to support referencing agent variables in tool form parameters. the full context behind this work is explained here: #13080.

Screenshots

Checklist

Important

Please review the checklist below before submitting your pull request.

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• [] I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

[crazywoola]

It seems some changes already included in #13165, please remove those in this PR to make sure only related code is changed.

[obadakhalili]

I'm a bit skeptical of defining variables_inputs here, because this method is used in contexts where variables_inputs isn't available.

[obadakhalili]

to render a form with an input that accepts variables, the schema is changed to accept inputs property, variables might be a better name, but it is confusing next to variable.

[obadakhalili]

Style check / Web Style (pull_request) fails because it asks me to use type instead of interface, which seems to be a new rule, but the main branch ins't adhering to this rule yet, so applying it here will make the diff too large.
I think it is better to handle the rule in the main branch and sync here after @crazywoola.