feat(regen): flux stt numerals and aura-2 multilingual tts voices (#746)

SDK regeneration for 2026-07-20 (Fern generator `5.14.18`, spec
`ff8fd2b7`), bumping the package to **7.5.1**.

## Features
- **Flux STT `numerals`** — new `types/listen_v2numerals.py`
(`Literal["true","false"]`) plus a `numerals` query param on
`listen.v2.connect()` and the raw client. Connection-time only. (docs
deepgram-docs#1020)
- Wire coverage added: `tests/custom/test_listen_v2_connect_wire.py`
(sync + async) pins that `/v2/listen` serializes `numerals=true` and
omits it when absent — the `/v2/listen` handshake previously had no wire
test.
  - Demonstrated in `examples/14-transcription-live-websocket-v2.py`.
- **New Aura-2 multilingual TTS voices** — ~40 voices added to
`SpeakV1Model` and `AudioGenerateRequestModel` (es/de/nl/fr/it/ja).
Additive only; not breaking.

## Breaking change from spec — mitigated (stays non-breaking)
- The spec removed `stt_latency` from `AgentV1LatencyReport`
(deepgram-docs#1006). `AgentV1LatencyReport` is a server-emitted
(read-only) message, so `stt_latency` is re-added by hand as a read-side
backward-compat field (frozen in `.fernignore`): `report.stt_latency`
keeps resolving to `None` instead of raising `AttributeError`, with **no
request/wire impact**. Covered by
`tests/custom/test_latency_report_stt_compat.py`. Keeps this a
minor/patch release, not a major.

## Manual patches reconciled
All 27 previously-frozen patches were still needed and re-applied
(byte-identical to `main`; the behavioral files don't appear in the net
diff). Intended changes to frozen files: `client_wrapper.py` version
annotations -> 7.5.1, and root + `types` `__init__.py` gained the new
`ListenV2Numerals` export alongside the legacy aliases. New frozen
entries: the two `agent_v1latency_report.py` shims and the two new
custom tests. All `.bak` files deleted.

## Verification
- `mypy src/deepgram` — clean (857 files)
- `pytest` — 331 passed, 1 skipped
- `ruff check src/deepgram` — clean (pre-existing warnings in
hand-maintained `tests/manual` + `tests/custom` only, identical on
`main`)

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Co-authored-by: fern-api[bot] <115122769+fern-api[bot]@users.noreply.github.com>