refactor(handlers): replace positional handler signature with context object pattern

[smithrashell]

Problem

Every message handler in src/background/handlers/ is called with four positional arguments:

// messageRouter.js
handler(request, dependencies, sendResponse, finishRequest)

Of the 7 handler files, only 2 use dependencies at all (sessionHandlers.js and onboardingHandlers.js, 5 specific functions). Every other handler either ignores it silently or marks it _dependencies. Several handlers also ignore _request. This means 30+ handler functions carry parameters purely for positional reasons, not because they need them.

The root issue is positional coupling — all handlers must accept all four args in the correct order even if they only need one or two.

Proposed Fix — Option 1: Context Object

Replace the positional call in messageRouter.js with a single context object:

// messageRouter.js — before
handler(request, dependencies, sendResponse, finishRequest)

// messageRouter.js — after
handler({ request, sendResponse, finishRequest })

Each handler destructures only what it needs:

// Handler that needs everything
export function handleGetOrCreateSession({ request, sendResponse, finishRequest }) { ... }

// Handler that only needs two things — no unused params at all
export function handleGetAllProblems({ sendResponse, finishRequest }) { ... }

// Handler that needs nothing from request
export function handleProblemSubmitted({ sendResponse, finishRequest }) { ... }

The 5 handlers in sessionHandlers and onboardingHandlers that currently pull from dependencies would instead import withTimeout, cleanupStalledSessions, checkOnboardingStatus, and completeOnboarding directly — these are all stable exported functions, not runtime values.

Why This Pattern

request, sendResponse, and finishRequest are runtime values that arrive per-message from Chrome's listener — they must be passed through. A context object is the right vehicle for them because:

• No positional ordering bugs (sendResponse and finishRequest are easy to transpose silently)
• Handlers declare only what they actually use — no _ noise
• Adding new context (e.g. sender, tabId) requires no signature changes in handlers that don't need it

This pattern is already used elsewhere in the codebase — useFloatingHintHandlers.js and several hook files use clean object destructuring for exactly this reason.

Files Affected

File	Unused param instances
src/background/handlers/problemHandlers.js	8 handlers
src/background/handlers/dashboardHandlers.js	~14 handlers
src/background/handlers/storageHandlers.js	~15 handlers
src/background/handlers/strategyHandlers.js	6 handlers
src/background/handlers/hintHandlers.js	4 handlers
src/background/handlers/sessionHandlers.js	~10 handlers (3 actually use deps)
src/background/handlers/onboardingHandlers.js	~8 handlers (2 actually use deps)
src/background/messageRouter.js	call site

Other Areas That Could Benefit

The same pattern could clean up unused parameters in:

• src/background/messageRouter.js switch cases — several inline handlers repeat the same pattern
• React component callback props — any (value, _index, _array) => style callbacks in JSX files where only the first arg is used
• Service functions with option bags — any service method taking 3+ positional config args could use an options object

Notes

• withTimeout and cleanupStalledSessions are exported from backgroundHelpers.js and can be imported directly
• checkOnboardingStatus and completeOnboarding are already imported in background/index.js from onboardingService.js
• The sessionHandlers tests mock withTimeout via deps — these would need updating to use jest.mock after the refactor