test: add unit tests across schema, lib, and cli modules

[notgitika]

Description

Add comprehensive unit tests across schema, lib, and cli modules to improve test coverage from to ~17% line coverage (874 tests total).

Test files added:

• Schema validators (7): agent-env, agentcore-project, aws-targets, deployed-state, mcp, mcp-defs, constants
• Lib utilities (6): credentials, env, subprocess, platform, zod, aws-account
• Lib packaging (5): helpers, errors, uv, node, python
• CLI cloudformation (4): stack-discovery, outputs-extended, logical-ids, bootstrap
• CLI operations (3): schema-mapper, preflight, create-mcp
• CLI TUI (8): useListNavigation, useTextInput, commands, process, diff, gradient, naming, timing

Minor source changes for testability:

• Export extractResult, parseSSELine, parseSSE from agentcore.ts
• Export findNextEnabledIndex extracted from useListNavigation.ts
• Export findPrevWordBoundary, findNextWordBoundary from useTextInput.ts
• Export extractNodeVersion (renamed from _extractNodeVersion) from node.ts
• Export extractPythonVersion from python.ts

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Other (please describe): Improving test coverage

Testing

How have you tested the change?

• I ran npm run test:all
• I ran npm run typecheck
• I ran npm run lint
• If I modified src/assets/, I ran npm run test:update-snapshots and committed the updated snapshots

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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terms of your choice.

[github-actions]

Coverage Report

Status	Category	Percentage	Covered / Total
🔵	Lines	26.9%	1720 / 6393
🔵	Statements	26.25%	1784 / 6794
🔵	Functions	26.36%	343 / 1301
🔵	Branches	23.48%	879 / 3743

Generated in workflow #355 for commit 97dc64a by the Vitest Coverage Report Action