bug: resolved model config tools and systemInstruction are overwritten

## What happened?

`GeminiChat` resolves a `GenerateContentConfig` from the model configuration service and then silently overwrites any resolved `systemInstruction` and `tools` values with the chat instance's fields.

In `packages/core/src/core/geminiChat.ts:781-798`:

1. `modelConfigService.getResolvedConfig()` returns `newConfig`.
2. `newConfig` is assigned to `currentGenerateContentConfig`.
3. The final config spreads that object, then unconditionally assigns `systemInstruction: this.systemInstruction` and `tools: this.tools`.

The adjacent TODO explicitly notes that these fields should not be overwritten when supplied through config. There is currently no targeted test covering this precedence behavior.

## How can this be reproduced?

1. Configure a model alias/resolved model config with a distinctive `systemInstruction` or `tools` value.
2. Send a message through `GeminiChat` using that model configuration.
3. Inspect the `GenerateContentConfig` passed to the content generator.
4. The configured value has been replaced by the chat-level value.

The same behavior occurs when a retry switches models and resolves a new model-specific configuration.

## What did you expect to happen?

Configuration precedence should be explicit and consistent. Fields supplied by a resolved model configuration should either be preserved or intentionally merged with chat-level fields, rather than being silently discarded.

## Why this matters

This makes parts of the model-configuration contract ineffective. Specialized aliases can unexpectedly lose model-specific tools or instructions, and the behavior is difficult to diagnose because configuration resolution itself succeeds.

## Suggested direction

- Define precedence/merge semantics for chat invariants versus resolved model configuration.
- Preserve configured values where allowed, or reject unsupported fields during configuration validation.
- Add tests for initial requests and retry-driven model switches.

## Client information

Found by source audit on current `main`, commit `f47d6c6f7`. This is platform-independent core configuration behavior; `/about` output is not applicable.

Start in packages/core/src/core/geminiChat.ts:781-798 and inspect the adjacent TODO and getResolvedConfig() flow. Trace how the final GenerateContentConfig is passed to the content generator during initial requests and retry-driven model switches. Done means precedence or merge semantics are defined and covered by tests for both paths.

Written by the indexing model from the issue text.