v1.5.1: Gemini reasoning fix, NIM json_schema, truncation warning

Fixes a user-reported bug where Gemini 2.5 Flash returned ~30 tokens
regardless of how high max_tokens was set, hardens the response cache
against poisoning by truncated responses, and adds JSON mode
reliability improvements for NVIDIA NIM and all providers.

Fixed

Gemini 2.5 reasoning budget no longer eats max_tokens

Gemini 2.5 Flash and 2.5 Pro are reasoning models. With their default
thinking budget they burned 90-98% of max_tokens on internal reasoning
before producing visible text. A caller asking for max_tokens=1000
routinely got back 30-40 tokens with finish_reason=length.

The Gemini provider adapter now injects a per-model default
reasoning_effort when the caller does not set one:

• gemini-2.5-flash defaults to "none" (accepts zero thinking budget,
  returns the full requested output)
• gemini-2.5-pro defaults to "low" (the minimum Google accepts for
  this model, which requires a non-zero thinking budget)

Clients that want full reasoning can pass reasoning_effort: "high"
explicitly. The adapter also normalizes the output budget onto
max_completion_tokens only and deletes max_tokens from the
outgoing request, because Gemini returns 400 when both are present.

Verified against the live Gemini API: the same prompt that produced
37 tokens before now returns 670+ tokens with finish_reason=stop.

Deprecated Gemini 2.0 models removed from catalog

gemini-2.0-flash and gemini-2.0-flash-lite both returned 404 "no
longer available to new users" from the live API. Removed from the
model list so callers cannot pick a dead model.

Response cache no longer stores truncated responses

When any choice in the upstream response carries
finish_reason=length, the cache now refuses to store it. Previously
a single truncated response from a reasoning model would pin the
bad answer for the entire TTL window (default 1 hour), causing the
"sometimes fails" pattern the user reported.

Cache key expanded to prevent cross-shape collisions

The cache key now includes tools, tool_choice, parallel_tool_calls,
response_format, reasoning_effort, seed, max_completion_tokens,
presence_penalty, and frequency_penalty. Previously two requests
with the same prompt but different tool definitions or response
formats would share a cache entry.

Added

NVIDIA NIM json_schema translation

NIM's OpenAI-compat endpoint does not support
response_format: { type: "json_schema" }. It requires the schema
in a vendor-specific nvext.guided_json field. The NIM provider
adapter now translates the standard parameter into the NIM-native
format automatically and removes the unsupported response_format
field from the outgoing request. json_object mode and requests
without response_format pass through untouched.

JSON truncation warning header

When a JSON-mode request (json_object or json_schema) hits
max_tokens and the output is almost certainly broken mid-token,
the response now carries:

X-FreeLLM-Warning: json-possibly-truncated

The caller knows immediately that the JSON is likely incomplete
without needing to attempt a parse.

reasoning_effort accepted in the request schema

The Zod schema now accepts reasoning_effort: "none" | "low" | "medium" | "high" so clients can override the per-model default.
Matches Gemini's OpenAI-compat knob and OpenAI's o-series
reasoning parameter.

finish_reason surfaced in the request log

The request log entry now carries a finishReason field populated
from the upstream response. When the reason is "length" the router
also emits a pino warning tagged with provider, model, max_tokens,
and reasoning_effort so operators can see which requests hit the
token cap.

Configuration

New environment variable:

Variable	Default	What it does
reasoning_effort (request field)	Per-model	"none" for gemini-2.5-flash, "low" for gemini-2.5-pro. Clients can override per request.

Tests

262 passing across 22 files (up from 215 at v1.5.0):

• gemini-provider.test.ts 14 tests for per-model defaults, mapRequest
  normalization, and catalog verification
• cache.test.ts 17 tests for isCacheable, truncation skip, and key
  discrimination across all request shape fields
• nim-provider.test.ts 6 tests for json_schema-to-nvext translation,
  passthrough, and field preservation
• json-truncation.test.ts 4 e2e tests for the warning header
• schema-tools.test.ts 2 new tests for reasoning_effort
• router.test.ts 4 new tests for finish_reason handling and
  cache anti-poisoning

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