Add 'Fail' mode when no shard selected

[Copilot]

This implements a "Fail" mode when no shard is selected, equivalent to pgcat's MR #859. When enabled, applications must explicitly select a shard for every connection, preventing data compromise in multi-tenant scenarios.

Problem

In multi-tenant applications using shards as tenants, it's critical that every query explicitly selects the correct shard. Without this requirement, queries may inadvertently access data from all shards, leading to potential data compromise and cross-tenant data leakage.

Solution

Added a new configuration option require_shard_selection that enforces explicit shard selection:

[general]
require_shard_selection = true

When enabled, queries without explicit shard selection will fail with a ShardSelectionRequired error.

Valid shard selection methods

• Comments: /* pgdog_shard: 1 */ SELECT * FROM users
• SET statements: SET pgdog.shard TO 1; SELECT * FROM users
• Sharding keys: /* pgdog_sharding_key: user_123 */ SELECT * FROM users

Implementation details

• Added require_shard_selection: bool configuration option to [general] section
• Added ShardSelectionRequired error variant to the parser error enum
• Modified query parser to check for explicit shard selection and fail if required but not provided
• Added comprehensive test coverage for the new functionality
• Ensured backward compatibility (default is false)

Example

# pgdog.toml
[general]
require_shard_selection = true

[[databases]]
name = "myapp"
shard = 0
host = "127.0.0.1"
port = 5432

[[databases]]  
name = "myapp"
shard = 1
host = "127.0.0.1"
port = 5433

With this configuration:

• ❌ SELECT * FROM users → fails with ShardSelectionRequired
• ✅ /* pgdog_shard: 1 */ SELECT * FROM users → routes to shard 1
• ✅ SET pgdog.shard TO 0; SELECT * FROM users → routes to shard 0

Testing

All existing tests pass and new tests verify the functionality works correctly with various query types and shard selection methods.

Fixes #221.

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