fix(core): resolve MCP union ambiguity with discriminator

[Nook127]

Issue for this PR

Closes #35359

Type of change

• Bug fix

What does this PR do?

When MCP config entries include enabled or environment keys, all CLI commands exit 1 with zero stdout/stderr. The root cause is a non-discriminated Schema.Union([ConfigMCPV1.Info, Schema.Struct({ enabled: Schema.Boolean })]) in the mcp field of ConfigV1.Info. With errors: "all" mode in ConfigParse.schema, all union members are evaluated. When an entry has enabled: true (also an optional field on Local), both members match the input, creating an ambiguity that Effect Schema 4.0.0-beta.83 reports as a parse error. This throws an InvalidError inside Effect.fnUntraced, becoming an uncaught defect that kills the process.

Added { discriminator: "type" } to the outer Schema.Union. The discriminator routes entries with type: "local"/"remote" unambiguously to ConfigMCPV1.Info, while entries without type fall through to { enabled: boolean } (used for config-layering disable overrides).

How did you verify your code works?

The change is a one-line addition of { discriminator: "type" } annotation to an existing Schema.Union, following the exact same pattern already used by ConfigMCPV1.Info (line 62 of mcp.ts). The discriminator makes the union unambiguous: entries with type route exclusively to ConfigMCPV1.Info, entries without type fall through to { enabled: boolean }. The existing isMcpConfigured check ("type" in entry) in the MCP service remains correct since the decoded output shape is unchanged.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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