Fix swarm context variable function injection

[marklysze]

Why are these changes needed?

Registering functions on an agent that goes into a swarm has different implementations based on whether you use the agent's functions parameter when creating or the register_for_llm decorator. This can cause issues in that using the functions parameter does not create a Tool as such, exceptions were being raised.

This aligns them so they both end up creating a Tool and have simplified the approach to hiding parameters on tool schemas for LLMs (essentially uses the same as Dependency Injection).

Related issue number

Closes #1157

Checks

• I've included any doc changes needed for https://docs.ag2.ai/. See https://docs.ag2.ai/docs/contributor-guide/documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
autogen/agentchat/conversable_agent.py	83.33%	1 Missing ⚠️

Files with missing lines	Coverage Δ
autogen/tools/dependency_injection.py	83.80% <100.00%> (+46.66%)	⬆️
autogen/agentchat/conversable_agent.py	67.47% <83.33%> (+50.65%)	⬆️

... and 134 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.