Make TypeScript sources compatible with --erasableSyntaxOnly

[Copilot]

Summary

Removes all TypeScript-only syntax extensions that emit runtime code, so the project can be built/typechecked with --erasableSyntaxOnly (and consumed by runtimes like Node's native TS stripping). The option is enabled in tsconfig.base.json to lock this in.

• Enums → as const object + type alias. Preserves Foo.Bar member access and runtime values. Applied to the public ClickHouseLogLevel and to internal/test enums (TestEnv, TestConnectionType, local test enums in select_query_binding.test.ts).
• Constructor parameter properties → explicit field declarations + assignments. Applied to TupleParam, SettingsMap, LogWriter, WebConnection, web/node ResultSet, NodeBaseConnection, SocketPool, and the BufferReader test helper.
• Reverse enum lookup. The old ClickHouseLogLevel[this.logLevel] (used for the startup log message) is replaced by a derived ClickHouseLogLevelName reverse map; output is unchanged.
• tsconfig.base.json: "erasableSyntaxOnly": true.

Example of the enum transform — call sites such as ClickHouseLogLevel.TRACE continue to work:

export const ClickHouseLogLevel = {
  TRACE: 0, DEBUG: 1, INFO: 2, WARN: 3, ERROR: 4, OFF: 127,
} as const
export type ClickHouseLogLevel =
  (typeof ClickHouseLogLevel)[keyof typeof ClickHouseLogLevel]

No namespaces or import = require(...) exist in the tree, so nothing else needed to change.

Checklist

• A human-readable description of the changes was provided to include in CHANGELOG

CHANGELOG

• Internal: TypeScript sources are now compatible with the --erasableSyntaxOnly compiler option. ClickHouseLogLevel is now exported as a const object plus a union type instead of a TS enum; runtime values and ClickHouseLogLevel.TRACE-style member access are unchanged.

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