feat(auth): add per-call AWS profile override middleware

[benjstoll]

Adds ProfileOverrideMiddleware that allows routing individual tool calls through dedicated per-profile MCP connections via a profile argument. Enabled with --allow-switch-profile CLI flag restricted to an explicit allowlist of profile names.

Summary

Changes

• Added ProfileOverrideMiddleware in mcp_proxy_for_aws/middleware/profile_switcher.py that intercepts a profile argument on any tool call,
  validates it against an allowlist, and routes the request through a dedicated per-profile MCP client with its own SigV4-signed transport.
• Added --allow-switch-profile CLI argument in cli.py that accepts one or more AWS profile names to enable the middleware.
• Wired the middleware into server.py with proper lifecycle management (lazy client creation, graceful shutdown of per-profile connections
  in the finally block).
• Added 12 unit tests covering tool schema injection, pass-through behavior, disallowed profiles, argument stripping, connection/tool-call
  error handling, and client disconnect logic. 90% branch coverage on the new middleware.
• Updated README.md with the new parameter in the configuration table and a "Multi-account access" section explaining how
  --allow-switch-profile interacts with --profile, including a JSON config example.

User experience

Before: Users who needed to query AWS resources across multiple accounts had to run separate proxy instances per profile, or manually
restart the proxy with a different --profile value.

After: Users pass --allow-switch-profile profile-a profile-b alongside their default --profile. Any tool call can include a profile
argument to route that single request through a dedicated connection signed with the specified profile's credentials. Tool calls without
profile continue to use the default connection. Each profile's connection is created lazily on first use, so there is no startup cost for
unused profiles.

Checklist

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• I have reviewed the contributing guidelines
• I have performed a self-review of this change
• Changes have been tested
• Changes are documented

Is this a breaking change? (Y/N)

• Yes
• No

Please add details about how this change was tested.

• Did integration tests succeed?
• If the feature is a new use case, is it necessary to add a new integration test case?

Acknowledgment

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