fix: auth plugin loginAs injection broken in 4.x

[DavertMik]

Summary

• Fix Container.support(name) to return plugin-injected functions directly instead of wrapping them in a non-callable Proxy. The 4.x ESM migration changed this method to always go through the proxy's lazyLoad(), which creates new Proxy({}, {...}) — designed for page object property access, not for calling functions. This broke the auth plugin's loginAs/login injection.
• Fix proxy get and getOwnPropertyDescriptor handlers to also return functions directly (covers destructuring paths like container.support()['loginAs']).
• Fix inject.js missing-key detection: !objects[key] never fired because lazyLoad() always returns a truthy Proxy. Changed to !(key in objects) which uses the proxy's has trap.

Root Cause

Container.support(name) was changed from:

// 3.x — works
return container.support[name] || container.proxySupport[name]

to:

// 4.x — broken for functions
return container.proxySupport[name]

The proxy's lazyLoad() returns new Proxy({}, { get... }) which has no apply trap and target {} is not callable. Page objects work fine (they use property access), but injected functions (like auth's loginAs) need to be called directly.

Test plan

• New unit test: function injected via container.append({ support }) is returned as a callable function
• All 33 container unit tests pass
• Full unit test suite passes (7 pre-existing AI SDK failures unrelated)
• Manual verification with auth plugin config using inject: "loginAs"

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