chore: lint fixes part 5

[pawbana]

• errname (3): UnknownRoute → ErrUnknownRoute; errorResponse → chatCompletionResponseError (chatcompletions); ErrorResponse → messagesResponseError (messages)
• slog field names (4): "%+v" nested slog.F → direct value; requestPath/upstreamPath → request_path/upstream_path (snake_case)
• slog message length (5): "failed to flush" → "failed to flush event stream"; "stream failed" → "stream processing failed" (2x); "unknown error" → "unknown stream error encountered" (2x)
• slog message case (1): "MCP client initialized" → "mcp client initialized"
• gosec G115 (4): added nolint:gosec for test constant uint32→int32 conversions
• errcheck (1): explicitly discarded RecordInterceptionEnded return value
• ineffassign (3): propagated streamCtx.Err() to interceptionErr instead of dead lastErr; added nolint:ineffassign for intentional prompt/promptFound resets

[pawbana]

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• feat: add lint check to CI cheks #263
• chore: lint fixes part 10 #262
• chore: lint fixes part 9 #261
• chore: lint fixes part 8 #260
• chore: lint fixes part 7 #259
• chore: lint fixes part 6 #258
• chore: lint fixes part 5 #257 👈 (View in Graphite)
• chore: lint fixes part 4 #256
• chore: lint fixes part 3 #255
• chore: lint fixes part 2 #254
• chore: lint fixes part 1 (automatic fixes) #253
• feat: extend golang lint check #252
• main

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