feat(platform): dry-run execution mode with LLM block simulation (#12483)

## Why

Agent generation and building needs a way to test-run agents without
requiring real credentials or producing side effects. Currently, every
execution hits real APIs, consumes credits, and requires valid
credentials — making it impossible to debug or validate agent graphs
during the build phase without real consequences.

## Summary

Adds a `dry_run` execution mode to the copilot's `run_block` and
`run_agent` tools. When `dry_run=True`, every block execution is
simulated by an LLM instead of calling the real service — no real API
calls, no credentials consumed, no side effects.

Inspired by
[Significant-Gravitas/agent-simulator](https://github.com/Significant-Gravitas/agent-simulator).

### How it works

- **`backend/executor/simulator.py`** (new): `simulate_block()` builds a
prompt from the block's name, description, input/output schemas, and
actual input values, then calls `gpt-4o-mini` via the existing
OpenRouter client with JSON mode. Retries up to 5 times on JSON parse
failures. Missing output pins are filled with `None` (or `""` for the
`error` pin). Long inputs (>20k chars) are truncated before sending to
the LLM.
- **`ExecutionContext`**: Added `dry_run: bool = False` field; threaded
through `add_graph_execution()` so graph-level dry runs propagate to
every block execution.
- **`execute_block()` helper**: When `dry_run=True`, the function
short-circuits before any credential injection or credit checks, calls
`simulate_block()`, and returns a `[DRY RUN]`-prefixed
`BlockOutputResponse`.
- **`RunBlockTool`**: New `dry_run` boolean parameter.
- **`RunAgentTool`**: New `dry_run` boolean parameter; passes
`ExecutionContext(dry_run=True)` to graph execution.

### Tests

11 tests in `backend/copilot/tools/test_dry_run.py`:
- Correct output tuples from LLM response
- JSON retry logic (3 total calls when first 2 fail)
- All-retries-exhausted yields `SIMULATOR ERROR`
- Missing output pins filled with `None`/`""`
- No-client case
- Input truncation at 20k chars
- `execute_block(dry_run=True)` skips real `block.execute()`
- Response format: `[DRY RUN]` message, `success=True`
- `dry_run=False` unchanged (real path)
- `RunBlockTool` parameter presence
- `dry_run` kwarg forwarding

## Test plan
- [x] Run `pytest backend/copilot/tools/test_dry_run.py -v` — all 11
pass
- [x] Call `run_block` with `dry_run=true` in copilot; verify no real
API calls occur and output contains `[DRY RUN]`
- [x] Call `run_agent` with `dry_run=true`; verify execution is created
with `dry_run=True` in context
- [x] E2E: Simulate button (flask icon) present in builder alongside
play button
- [x] E2E: Simulated run labeled with "(Simulated)" suffix and badge in
Library
- [x] E2E: No credits consumed during dry-run