refactor(core): reuse base environment config

[chenjiahan]

Summary

This PR reuses a shared base environment config when initializing Rsbuild environments, keeping the default and named environment paths aligned while avoiding duplicate construction of the same base config.

[Copilot]

Pull request overview

This PR refactors environment initialization in @rsbuild/core to reuse a single shared “base environment” config object, keeping the default-environment path and named-environments path aligned while avoiding duplicated base-config construction.

Changes:

• Introduce a baseEnvironmentConfig composed from baseConfig plus a dev subset (pick(dev, allowedEnvironmentDevKeys)).
• Reuse baseEnvironmentConfig when merging named environment configs via mergeRsbuildConfig.
• Reuse baseEnvironmentConfig when constructing the default environment config.

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[gemini-code-assist]

Code Review

This pull request refactors initEnvironmentConfigs in packages/core/src/initConfigs.ts by extracting a shared baseEnvironmentConfig object to reduce code duplication. Feedback from the review suggests removing a redundant object spread around mergeRsbuildConfig for better clarity. Additionally, it is recommended to use mergeRsbuildConfig instead of a shallow spread for the default environment configuration to avoid side effects caused by mutating nested properties in the base configuration.

[gemini-code-assist]

The object spread around mergeRsbuildConfig is redundant because mergeRsbuildConfig already returns a new merged object. Removing the extra spread and braces simplifies the code.

          const environmentConfig = mergeRsbuildConfig(
            baseEnvironmentConfig,
            config,
          ) as unknown as MergedEnvironmentConfig;

[gemini-code-assist]

Using a shallow spread { ...baseEnvironmentConfig } means that nested objects like source and output are shared with the original normalizedConfig. Since applyEnvironmentDefaultConfig mutates these nested properties (e.g., config.source.entry), it leads to side effects on the base configuration passed to this function. For consistency with the multi-environment path and to ensure the base configuration remains immutable, consider using mergeRsbuildConfig to create a fresh copy.

    [defaultEnvironmentName]: applyEnvironmentDefaultConfig(
      mergeRsbuildConfig(baseEnvironmentConfig) as MergedEnvironmentConfig,
    ),