[modem]: Add ability to change ESP_MODEM_C_API_STR_MAX from Kconfig (IDFGH-13667)

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Description

This PR introduce ability to change ESP_MODEM_C_API_STR_MAX from Kconfig (idf.py menuconfig).

In esp_modem in C-API there is ESP_MODEM_C_API_STR_MAX which defines max size of AT response returned to user.
There is no obvious way to increase this size. Some workaround was found in #184, but I think use Kconfig option is more clear and straightforward.

Related

#184
#624

Testing

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Checklist

Before submitting a Pull Request, please ensure the following:

• 🚨 This PR does not introduce breaking changes.
• All CI checks (GH Actions) pass.
• Documentation is updated as needed.
• Tests are updated or added as necessary.
• Code is well-commented, especially in complex areas.
• Git history is clean — commits are squashed to the minimum necessary.

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