.Net - Update and Fixes for AzureAIAgent - #12348

### Motivation and Context
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Address customer input and discovered issues for `AzureAIAgent`

### Description
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These notes will help understanding how your code works. Thanks! -->

Address several issues:

- Support image input
- Null reference when function calling parameter is null
- Null reference when using BingGroundingTool with streaming
- Add samples for response-format
- Fix connection resolution for declarative bing tool

Fixes: microsoft#12288

### Contribution Checklist
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- [X] The code builds clean without any errors or warnings
- [X] The PR follows the [SK Contribution
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raises no violations
- [X] All unit tests pass, and I have added new tests where possible
- [X] I didn't break anyone 😄

…ADME (microsoft#14699)

## Motivation

The sample tables in `dotnet/samples/GettingStartedWithAgents/README.md`
contain 20 broken links:

1. **18 links use a wrong path prefix** — they were written as
`./dotnet/samples/GettingStartedWithAgents/...`, but the README itself
already lives in `dotnet/samples/GettingStartedWithAgents/`, so the
links resolve to a doubled path and 404 on GitHub. The BedrockAgent,
CopilotStudioAgent and Orchestration sections in the same file use
correct relative paths (e.g. `./BedrockAgent/...`), which makes the
inconsistency clear.
2. **2 links point to renamed files** in the `Orchestration/` folder:
`Step02a_Sequential.cs` is now `Step02a_SequentialCancellation.cs` and
`Step04b_HandoffWithStructuredInput.cs` is now
`Step04a_HandoffWithStructuredInput.cs`.
3. **1 entry points to a removed sample**: `Step03_AzureAIAgent_Chat.cs`
was removed and replaced by `Step03_AzureAIAgent_Vision.cs` in microsoft#12348
(both changes in the same commit), but the README row was never updated.

## Changes

All in `dotnet/samples/GettingStartedWithAgents/README.md` (20 lines):

- Agent section (7 links): strip the wrong
`./dotnet/samples/GettingStartedWithAgents/` prefix.
- OpenAIAssistant section (5 links): same prefix fix.
- AzureAIAgent section (6 links): same prefix fix; additionally the
Step03 row now points to `./AzureAIAgent/Step03_AzureAIAgent_Vision.cs`
with its title and description updated to match the replacement sample.
- Orchestration section (2 links): update to the renamed
`Step02a_SequentialCancellation.cs` and
`Step04a_HandoffWithStructuredInput.cs`.

## Verification

- Every new link target was verified to exist on the current `main` tree
(file listing of `dotnet/samples/GettingStartedWithAgents/`,
`OpenAIAssistant/`, `AzureAIAgent/`, `Orchestration/`).
- The removal of `Step03_AzureAIAgent_Chat.cs` and addition of
`Step03_AzureAIAgent_Vision.cs` were confirmed via the commit history of
the `AzureAIAgent` folder (microsoft#12348); the new description mirrors the
wording of the OpenAIAssistant Vision row.
- No text outside the affected table rows was modified.

> [!NOTE]
> This documentation fix was prepared with AI assistance (an automated
contribution workflow). Happy to adjust anything — e.g. if you prefer
keeping the old `Step03_AzureAIAgent_Chat` row title or handling it
differently.