Expose the resolved File Search retrieved chunks (not only cited ones) on the file_search_result step

**Environment:** `google-genai` 2.10.0 · Python 3.13 · macOS · Gemini API (not Vertex) · model `gemini-3.5-flash` with the `file_search` tool.

### Summary

When using File Search via the Interactions API, there is no way to inspect the **full set of chunks the retriever returned and injected into the model's context**. The retrieved chunk text is observable *only* indirectly, via `model_output` → `file_citation.source` annotations — i.e. only for chunks the model ends up **citing**. This blocks reliability/eval work and debugging "what context did the model actually see."

### What's observable today

- `file_citation` annotations expose `source` (chunk text), `document_uri`, `file_name`, `custom_metadata`, `start_index`/`end_index`. **Only for cited chunks.** (Side note: the `source` field appears undocumented — the public File Search docs still describe annotations as metadata-only.)
- The `file_search_result` step contains only `call_id`, `type`, and an opaque base64 `signature` (~16KB, encrypted) — no readable retrieved content.
- `file_search_stores.documents.get`/`list` return metadata only; `download_media` on a document returns `403 PERMISSION_DENIED`.
- Token usage doesn't help: `total_input_tokens` stays flat and `total_tool_use_tokens` is ~constant regardless of `top_k`, so the retrieved-context size isn't reflected anywhere.

### Why it matters

Rigorous RAG faithfulness/groundedness evaluation must judge an answer against the **actual retrieved context**, not the full source document. Today we can only approximate it with the *cited* chunks. Empirically, at the default `top_k` the model cites every retrieved chunk (cited count tracks `top_k` 1:1 up to ~5), so cited ≈ retrieved in that regime — but this is **inferred**, not guaranteed, and it breaks down at higher `top_k` (e.g. `top_k=10` → 8 cited, `top_k=20` → 16 cited), where retrieved-but-uncited chunks become invisible. (Same concern raised re: server-side context being a black box: https://x.com/_philschmid/status/2069458803074986376)

### Repro

```python
store = "fileSearchStores/"
it = client.interactions.create(
model="gemini-3.5-flash",
input="",
tools=[{"type": "file_search", "file_search_store_names": [store], "top_k": 20}],
store=True,
)
full = client.interactions.get(it.id)
# file_search_result step: only call_id / type / signature — no retrieved chunks.
# Retrieved chunk text is reachable only via model_output file_citation.source,
# i.e. only the CITED chunks; with a large top_k the cited count is < top_k, so
# the retrieved-but-uncited chunks are invisible.
```

Surface the resolved File Search retrieval on the `file_search_result` step (or via an opt-in flag): the **full set of retrieved chunks** — text + document/source ref + relevance score, **including uncited ones**. This makes the retrieved context inspectable for evaluation and debugging, and lets developers trim/manage it as suggested in the GA guidance.