Refactor: Replace stateful set_active_account with per-tool account_id parameter

[mattzcarey]

Problem

The current account selection flow requires a stateful two-step dance:

1. Call accounts_list to discover accounts
2. Call set_active_account to persist the selection in a Durable Object

This breaks in many MCP clients that don't maintain state across tool calls, leading to repeated "No currently active accountId" errors. Multiple issues have been filed about this: #226, #260, #281, #175.

Additional problems with the current approach:

• set_active_account is not registered for account-scoped tokens, but error messages still reference it ([Bug] set_active_account tool not exposed - cannot use D1/KV/R2 tools with single account #260)
• Account persistence doesn't work across different MCP server instances (Account persistence issue: set_active_account succeeds but other MCP tools report "No currently active accountId" #226)
• Requires an extra round-trip before any useful work can happen

Proposed Solution

Add an optional account_id parameter to every tool that requires an account, and remove the stateful set_active_account tool entirely.

Spec

Resolution logic (resolveAccountId)

When a tool needs an account ID, resolve it in this order:

1. Account token: Always use props.account.id. Ignore any account_id parameter.
2. User token + account_id provided: Validate it against props.accounts. Use it if valid, return error listing valid accounts if not.
3. User token + single account + no account_id: Auto-select the only account.
4. User token + multiple accounts + no account_id: Return an error response listing available accounts (id + name) so the LLM can self-correct on the next call.

Tool changes

• Add account_id?: string (optional Zod param) to every tool that currently calls getActiveAccountId()
• Shared AccountIdParam schema and resolveAccountId() helper in a new packages/mcp-common/src/tools/account.helpers.ts
• Error responses include available_accounts list — no separate accounts_list call needed to recover

What gets removed

• set_active_account tool registration (from account.tools.ts)
• getActiveAccountId() / setActiveAccountId() from the CloudflareMcpAgent interface
• getActiveAccountId() / setActiveAccountId() implementations from all ~14 app classes
• active_account_id storage from UserDetails Durable Object
• MISSING_ACCOUNT_ID_RESPONSE constant

What stays

• accounts_list tool (for explicit discovery)
• UserDetails Durable Object class (may be used for other per-user state)

Affected tools (~20+ across packages/mcp-common and apps/)

packages/mcp-common/src/tools/: d1 (5), kv (5), r2 (4), workers (3), zones (2), hyperdrive (4)

apps/: ai-gateway, workers-observability, workers-builds, logpush, autorag, auditlogs, browser-rendering, dex-analysis, dns-analytics, graphql, radar, cloudflare-one-casb

Benefits

• Stateless: Works with any MCP client, no state management required
• Fewer round-trips: Single account users auto-resolve; multi-account users get account list in the error response
• Self-correcting: Error responses include available accounts, so LLMs can retry immediately with the right ID
• Simpler code: No Durable Object reads on every tool call, synchronous resolution from props