[Feat] feat(aliases): allow setting reasoning level

[Killusions]

Adds optional reasoning_effort support to static model aliases. The main use case is replacing non-reasoning models with reasoning ones without changing client behaviour: map the old alias to the new model with reasoning_effort=none and clients see no difference. It also makes it easy to expose multiple capability tiers (fast, thorough, etc.) from a single engine.

Aliases can be configured as alias:model|reasoning_effort=high (CLI/JSON) or as a dict with model and reasoning_effort keys (YAML). The router injects the configured value into forwarded requests unless the client already set one.

FIX #911

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Code Review

This pull request introduces an enhanced alias configuration system that allows for parameter overrides, specifically adding support for 'reasoning_effort'. It implements a new AliasConfig dataclass designed for backward compatibility with existing string-based aliases while enabling more complex configurations via CLI, JSON, or YAML. The routing logic has been updated to inject these configured parameters into backend requests if they are not already provided by the client. Review feedback focuses on improving the robustness of the alias string parsing to handle empty entries and ensuring that null values in YAML configurations are correctly handled to avoid validation errors.

[Killusions]

@nejch Could you maybe take a look at this?

[Killusions]

@ruizhang0101 Thank you for merging my bugfix, could you maybe also have a look here, I'm also open to alternate approaches, but we are currently using this patch internally and we'd really appreciate having some way to configure reasoning and especially disable it