Handle assistant messages with 'tool_calls' when used in chat_template

[Jeadie]

🗣 Description

• When using a chat_template that parse the tool call results (from an assistant message), the current implementation of MessageContent does not support the possible nesting.
• The core change here is

-pub type MessageContent = Either<String, Vec<IndexMap<String, String>>>;
+pub type MessageContent = Either<String, Vec<IndexMap<String, Value>>>;

• Also, change BasicProcessor to use template_action() -> MessagesAction::Keep (instead of MessagesAction::FlattenOnlyText) .

🔨 Related Issues

• Resolves issues found in RequestMessage with tool_calls in Assistant message. EricLBuehler/mistral.rs#793

[github-actions]

Code Metrics Report

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 Dockerfile              1           34           25            0            9
 Happy                   1          442          369            0           73
 JSON                   12          105          104            0            1
 Python                 52         2268         1930           69          269
 TOML                   20          625          559            2           64
 YAML                    2           21           19            2            0
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 Jupyter Notebooks       4            0            0            0            0
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 (Total)                            273          201           32           40
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 Markdown               38         2760            0         2094          666
 |- BASH                 6          103          100            0            3
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 (Total)                           3364          531         2094          739
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 Rust                  260        75712        68239         1548         5925
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 (Total)                          76929        68264         2665         6000
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 Total                 393        82002        71273         3715         7014
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