fix(cli): fix overrides write command to properly merge existing overrides

[Ryan-Amirthan]

Description

Link to Devin run: https://app.devin.ai/sessions/aae6845f5c3c457d93437d7748ee7a23
Requested by: ryanstep@buildwithfern.com (@Ryan-Amirthan)

Fixes three bugs in fern overrides write that prevented existing overrides from being merged when regenerating:

1. Typo in paths key: Was checking for "path" (singular) instead of "paths" (plural), causing existing path overrides to be ignored
2. Wrong schema location: Was reading schemas from existingOverrides.path instead of existingOverrides.components.schemas
3. Wrong output path: Was always deriving output filename from the spec file (e.g., openapi.json → openapi-overrides.json) instead of writing back to the configured overrides file

Changes Made

• Fixed paths key check from "path" to "paths" to match OpenAPI spec structure
• Fixed schemas reading to use existingOverrides.components?.schemas instead of existingOverrides.path
• When an overrides file is configured in generators.yml, now writes back to that same path
• When no overrides file is configured, defaults to .yml extension (not the spec's extension)

Testing

• Unit tests added/updated
• Manual testing completed (lint checks pass)

Human Review Checklist

• Verify the key names (paths, components.schemas) match standard OpenAPI override file structure
• Consider whether the existing e2e test at packages/cli/ete-tests/src/tests/write-overrides/writeOverrides.test.ts should be updated to cover the merge scenario

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