[Repo Assist] test(config): add tests for config_env.go gateway env var parsers (#5197)

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## Summary

`internal/config/config_env.go` contains five functions that parse
gateway configuration from environment variables:

- `GetGatewayPortFromEnv()` – parses `MCP_GATEWAY_PORT`
- `GetGatewayDomainFromEnv()` – reads `MCP_GATEWAY_DOMAIN`
- `GetGatewayAPIKeyFromEnv()` – reads `MCP_GATEWAY_API_KEY`
- `GetGatewayToolTimeoutFromEnv()` – parses `MCP_GATEWAY_TOOL_TIMEOUT`
- `toolTimeoutEnvOrDefault()` – fallback-to-default helper

None of these had any test coverage before this PR. These are gateway
entry-point validators and a bug here (e.g. accepting out-of-range ports
or sub-minimum timeouts) would silently misconfigure the server.

## Tests Added

**`TestGetGatewayPortFromEnv`** (8 cases)
- Not set → error
- Non-integer → error with "invalid MCP_GATEWAY_PORT value"
- `0`, `-1`, `65536` → out-of-range error
- `1`, `65535`, `3000` → valid, returns parsed int

**`TestGetGatewayDomainFromEnv`** (3 cases)
- Not set → `""`
- Domain only → returned as-is
- Domain + port → returned as-is

**`TestGetGatewayAPIKeyFromEnv`** (2 cases)
- Not set → `""`
- Set → returned as-is

**`TestGetGatewayToolTimeoutFromEnv`** (8 cases)
- Not set → `(0, false, nil)`
- Non-integer, `0`, `-1`, `9` → error
- `10` (minimum), `60` (default), `600` → `(val, true, nil)`

**`TestToolTimeoutEnvOrDefault`** (5 cases)
- Not set → `DefaultToolTimeout`
- Invalid string → `DefaultToolTimeout`
- Below-minimum (5) → `DefaultToolTimeout`
- Valid `120` → `120`
- Minimum-valid `10` → `10`

## Test Status

Tests are logically correct and follow the patterns established in this
codebase (`t.Setenv`, table-driven, `testify/assert` + `require`). Local
execution was not possible due to network restrictions in the CI sandbox
(Go module downloads blocked), but these tests are simple pure-logic
unit tests with no external dependencies beyond testify.




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Author: lpcox