fix: Vertex/Gemini whitespace crash + surface retry-after hints (0.15.8)

[nyo16]

Summary

Fixes a production crash in Vertex AI / Gemini response parsing and adds first-class support for server-suggested retry-after hints across all providers.

The crash

Vertex/Gemini occasionally return text parts whose content is only whitespace (e.g. "\n\n\n") — typically between tool calls or as filler when the model is partially blocked. Ecto's default :empty_values for cast/3 treats
whitespace-only strings as empty, so Nous.Message.ContentPart's changeset dropped the content field entirely and then raised:

** (Ecto.InvalidChangesetError) could not perform  because changeset is invalid.

Errors

    %{content: [{"content is required", []}]}

Params

    %{"content" => "\n\n\n", "type" => :text}

…taking down the whole Nous.LLM.run_with_tools/6 call from inside background jobs.

What this PR does

Fixes

• Nous.Message.ContentPart overrides :empty_values to [""] so legitimate whitespace content is preserved.
• Nous.Messages.Gemini.parse_content/1 defensively skips whitespace-only text parts (matching the existing guard in Nous.StreamNormalizer.Gemini).
• Nous.Messages.Gemini.parse_content/1 no longer silently drops nullary functionCall parts (those without args); pattern now requires only name and falls back to %{}.

Adds — retry-after surfacing

• New Nous.Errors.RetryInfo parses google.rpc.RetryInfo from Vertex/Gemini error bodies and the standard Retry-After header into milliseconds. Returns nil when the server gives no hint — itself meaningful for Google APIs,
  since long-term/daily quota exhaustion deliberately omits RetryInfo to discourage retry loops.

• Nous.Errors.ProviderError gains :retry_after_ms alongside the existing :status_code. Both fields are populated automatically by Nous.Provider.request/3 and request_stream/3 when the HTTP layer returns an error tuple.
  Callers can now branch on rate limits without parsing provider-specific bodies:

  case Nous.LLM.run_with_tools(...) do
    {:error, %Nous.Errors.ProviderError{retry_after_ms: ms}} when is_integer(ms) ->
      {:snooze, ms}                     # server-suggested delay
    {:error, %Nous.Errors.ProviderError{status_code: 429}} ->
      {:snooze, exp_backoff(attempt)}   # rate-limited, no hint
    ...
  end

Adds — Gemini diagnostics

• Nous.Messages.Gemini.from_response/1 captures finishReason and promptFeedback into message.metadata and emits a Logger.warning when content is empty for non-STOP reasons (SAFETY, RECITATION, MAX_TOKENS, etc.) or a
  prompt block. Previously these signals were discarded, so blocked generations manifested as silent empty messages.

Changes — HTTP error shape

• Nous.HTTP.Backend.Req, Nous.HTTP.Backend.Hackney, and Nous.HTTP.StreamBackend.Req now return {:error, %{status, body, headers}} instead of %{status, body}, surfacing response headers so Retry-After can be parsed.
  headers is a list of {name, value} tuples. Existing pattern matches on %{status: _, body: _} continue to work — map matching is non-exhaustive.

Changes — tool-call IDs

• Unified Gemini tool-call ID generation. The previous parse_content/1 used "gemini_#{:rand.uniform(10_000)}" (~50% birthday-paradox collision at ~118 calls) while parse_parts/1 used "call_#{:rand.uniform(1_000_000)}". Both
  now share a generate_tool_call_id/0 helper using 64 bits of :crypto.strong_rand_bytes/1, base64url-encoded.

Test plan

• mix test — 1674 tests, 0 failures (24 new tests).
• New Nous.Errors.RetryInfoTest — 16 tests covering body parsing (google.rpc.RetryInfo, fractional durations, missing/malformed entries), header parsing (Retry-After integer, case-insensitive, non-integer/HTTP-date
  rejection, negative values), precedence (body wins over header), and edge cases (non-map input, empty maps).
• Extended Nous.HTTP.BackendTest against both Req and Hackney via Bypass: verifies Retry-After header round-trips through RetryInfo, and that a Vertex-shaped 429 body with RetryInfo is parsed end-to-end.
• Extended Nous.ProviderTest: request/3 correctly populates :status_code from HTTP error tuples, :retry_after_ms from both body and header sources, and leaves both nil for transport errors.
• Regression tests in Nous.MessagesGeminiTest: whitespace-only text parts are skipped (the original Vertex "\n\n\n" payload), finishReason and promptFeedback land in metadata, functionCall without args is parsed.
• Regression test in Nous.Message.ContentPartTest: ContentPart.text("\n\n\n") no longer raises; nil and "" content are still rejected.
• mix compile --warnings-as-errors --force clean.

Migration notes

• HTTP error tuple shape change is additive. Code matching %{status: _, body: _} is unchanged. Code matching %{status: _, body: _} = err and reaching for Map.keys(err) will see an extra :headers key.
• ProviderError.retry_after_ms is new; existing pattern matches on the struct continue to compile because all fields default to nil.