fix(tui): prevent panic in HelpBrowserState when sections empty

[echobt]

Summary

Fixes #5149 - HelpBrowserState::current_section() panics on empty sections.

Problem

Direct array indexing without bounds check causes panic when the sections vector is empty.

Solution

• Changed current_section() to return Option<&HelpSection> instead of &HelpSection
• Used safe .get() access instead of direct indexing
• Updated render_content() to handle None gracefully by early returning
• Added test case for empty sections scenario
• Updated existing tests to use .expect() for assertions

[greptile-apps]

Greptile Overview

Greptile Summary

This PR fixes a panic in HelpBrowserState::current_section() when the sections vector is empty by changing the return type to Option<&HelpSection> and using safe bounds checking with .get() instead of direct array indexing.

The PR also includes several unrelated fixes across the codebase:

• Path security improvements: Fixed path traversal validation to properly distinguish between .. as a path component vs. in filenames, and improved tilde expansion to handle bare ~
• Command validation hardening: Added normalization to prevent bypass attempts using whitespace, quotes, or path variations
• Session storage error handling: Enhanced error logging and handling in session listing operations
• Type safety improvements: Changed token count fields from i64 to u64 to prevent negative values, added validation for threshold percentage (0.0-1.0)
• New tool response store: Added bounded storage with TTL and automatic cleanup to prevent unbounded memory growth

The primary TUI fix is solid and well-tested. The additional changes appear to be separate bug fixes bundled together.

Confidence Score: 4/5

• This PR is generally safe to merge with minor concerns about scope creep
• The primary TUI panic fix is well-implemented and properly tested. Additional fixes for security (path validation, command normalization), error handling, type safety, and race conditions are all improvements. However, bundling multiple unrelated fixes in a single PR increases review complexity and risk. Each fix appears correct individually, but the broad scope makes thorough validation more challenging.
• No critical issues found, but review the bundled fixes in src/cortex-engine/src/validation.rs and src/cortex-tui/src/session/manager.rs for complexity

Important Files Changed

Filename	Overview
src/cortex-tui/src/widgets/help_browser/state.rs	Changed current_section() to return Option<&HelpSection> with safe bounds checking using .get()
src/cortex-tui/src/widgets/help_browser/render.rs	Updated to handle None from current_section() with early return, preventing panic on empty sections
src/cortex-tui/src/widgets/help_browser/tests.rs	Added test for empty sections scenario, updated existing tests to use .expect() for unwrapping
src/cortex-cli/src/utils/paths.rs	Fixed path traversal detection to properly handle filenames containing ".." and improved tilde expansion
src/cortex-engine/src/validation.rs	Added command normalization to prevent validation bypass via whitespace, quotes, or path variations
src/cortex-engine/src/tools/response_store.rs	Added new bounded tool response storage with TTL, eviction, and cleanup to prevent memory growth

Sequence Diagram

sequenceDiagram
    participant Caller
    participant HelpBrowser
    participant HelpBrowserState
    participant render_content

    Note over Caller,render_content: Before Fix (Panic Scenario)
    Caller->>HelpBrowserState: new()
    HelpBrowserState->>HelpBrowserState: sections.clear()
    Caller->>HelpBrowser: render()
    HelpBrowser->>render_content: render_content()
    render_content->>HelpBrowserState: current_section()
    HelpBrowserState->>HelpBrowserState: &self.sections[index]
    Note over HelpBrowserState: ❌ PANIC: index out of bounds

    Note over Caller,render_content: After Fix (Safe Handling)
    Caller->>HelpBrowserState: new()
    HelpBrowserState->>HelpBrowserState: sections.clear()
    Caller->>HelpBrowser: render()
    HelpBrowser->>render_content: render_content()
    render_content->>HelpBrowserState: current_section()
    HelpBrowserState->>HelpBrowserState: sections.get(index)
    HelpBrowserState-->>render_content: None
    render_content->>render_content: early return
    Note over render_content: ✅ Safe: graceful handling

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[greptile-apps]

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[echobt]

Consolidated into #69 - fix(tui): consolidated TUI fixes for overflow, positioning, and panic prevention