--new-session doesn't create a new session when an existing conversation ID is present

## Goal
`azd ai agent invoke --new-session` results in a new session ID from the backend.

## Context
The backend derives sessions from a conversation ID, so when --new-session is used and an existing conversation ID is present in the .foundry-agent.json, the same session ID is returned by the server.

This is unexpected because we have `--new-session` and `--new-conversation` as separate flags.

To workaround this and always get a new session, you have to pass both `--new-session and --new-conversation`.

I think we can make these mutually exclusive. new session always does a new conversation too.

## Current Behavior
when --new-session is used and an existing conversation ID is present in the .foundry-agent.json, the same session ID is returned by the server.

## Desired Behavior
when --new-session is used it also starts a new conversation so that a new session ID is received.

## Implementation Notes (optional but powerful)
- `--new-session` automatically includes the behavior of `--new-conversation`, like if I do `--new-session --new-conversation` today.
- `--new-session --new-conversation` is still valid, its just not necessary anymore since `--new-session` will include it.
- `--new-conversation` is valid alone, and continues to use the same session.

## Acceptance Criteria
- [ ] Unit tests added or updated
- [ ] Existing test suite passes
- [ ] Lint or format passes

## How to Test
1. `azd ai agent init -m "https://github.com/microsoft-foundry/foundry-samples/blob/main/samples/python/hosted-agents/agent-framework/echo-agent/agent.yaml"`
2. `azd up`
3. `azd ai agent invoke "hello"`
4. `azd ai agent invoke "hi" --new-session`

Verify the session id between 3 and 4 are different.