Agents Sessions window: new-session chat input placeholder no longer rotates through random prompts

## Description

The new-session chat input in the Agents Sessions window has a `RANDOM_PLACEHOLDERS` list of friendly prompts (e.g. "What are you building?", "What will you ship today?", "Pitch your idea", etc.) that is meant to pick a random one per widget instance via `getRandomChatInputPlaceholder()` (`src/vs/sessions/contrib/chat/browser/newChatInput.ts`).

However, the primary new-session composer (`NewChatWidget`, in `src/vs/sessions/contrib/chat/browser/newChatWidget.ts`) always constructs `NewChatInputWidget` with an explicit fixed placeholder:

```ts
placeholder: localize('newSessionPromptPlaceholder', "Pitch your idea"),
```

Since `NewChatInputWidget`'s editor creation does:

```ts
placeholder: this.options.placeholder ?? getRandomChatInputPlaceholder(),
```

the explicit `placeholder` option always wins, so `getRandomChatInputPlaceholder()` never runs for this composer. As a result the new-session input is permanently stuck showing "Pitch your idea" and never rotates/varies, even though the rotation logic and the full list of prompt variants still exist in the code.

## Steps to Reproduce

1. Open the Agents Sessions window.
2. Open a new session / new chat composer multiple times (or reload the window).
3. Observe the placeholder text in the empty input box.

## Expected Behavior

The placeholder should vary between the different friendly prompts in `RANDOM_PLACEHOLDERS`, as it apparently used to.

## Actual Behavior

The placeholder is always "Pitch your idea" and never changes.

## Screenshot

Reported placeholder stuck on "Pitch your idea":

(see attached screenshot in the originating Slack thread)

---

_Created from a [Slack conversation](https://vscodeteam.slack.com/archives/C0AM349P43S/p1788381574958309?thread_ts=1788381574.958309&cid=C0AM349P43S)._