Introduces a process.on('unhandledRejection') handler in src/gemini.ts
as a workaround for an issue where 429 ClientErrors originating from
the @google/genai library's sendMessageStream during iteration can
cause an unhandled rejection, even when caught within local try/catch
blocks in the application code (e.g., in processGeminiStream).
The handler specifically identifies this known 429 ClientError based on
its type and message content. If matched, it logs a warning indicating
the known issue is being suppressed and prevents process.exit(1).
Any other genuinely unhandled promise rejections will still be logged
as critical errors and will terminate the application, maintaining
default behavior for unexpected issues. This workaround mitigates a
suspected library-internal problem related to error propagation during
asynchronous stream iteration.
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