Deduplicate and refactor modal packages

[AlexNPavel]

This commit addresses technical debt from the MCE interactive modals addition (PR #574). When the MCE modals were first introduced, code deduplication between the existing launch modals and new MCE modals was suggested during review, but was not implemented at the time due to the complexity of the new work.

This refactoring was performed using Claude Code to systematically identify and eliminate duplication across all modal packages.

Changes:

• Created pkg/slack/modals/common/ with 4 utility files (1,123 lines)
• Eliminated 2,199 lines of duplicate code across 32 files
• Standardized metadata construction using MetadataBuilder pattern
• Extracted common view builders (BuildSimpleView, BuildSelectModeView, etc.)
• Removed duplicate SubmissionView, PrepareNextStepView, and ResultView functions
• Refactored simple modals (auth, done, refresh, mce/list, mce/lookup) to use common patterns
• Standardized error context strings to gerund form without unnecessary articles

The common utilities provide:

• MetadataBuilder: Fluent API for consistent metadata construction
• RegisterSimpleModal: Factory for simple modal registration
• MakeSimpleProcessHandler: Handler factory for basic submission flows
• BuildSimpleView, BuildListResultModal: View builders for common patterns
• BuildSelectModeView, BuildFilterVersionView, BuildPRInputView, etc.: Shared view logic

Net result: -1,317 lines (-76% reduction in launch/views.go, -73% in mce/create/views.go) while maintaining all existing functionality and improving code maintainability.

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/test all

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@AlexNPavel: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/prow/security	edf6453	link	false	/test security
ci/prow/govulncheck	edf6453	link	false	/test govulncheck

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