fix: secrete manager may return undefined

[takumiyoshikawa]

In this PR, I'm improving the typing of the secret manager within the function service.

Below is an actual execution log from the vault on the Tailor platform. As shown, getSecret can potentially return undefined.
Also, since getSecrets takes an object with the vault key names as arguments, we can narrow the type accordingly. In terms of behavior, if a non-existent key is specified, the key will not be included in the response object. Therefore, the keys in the response of getSecrets should be treated as optional.

[Execution Logs]
2025-06-24 09:05:37.955922 +0000 UTC STARTED
2025-06-24T09:05:38.117075074Z INFO === getSecrets() behavior ===
2025-06-24T09:05:38.117334445Z INFO Result object: [Object: null prototype] { "existing-key": "this-is-the-secret-value" }
2025-06-24T09:05:38.117350195Z INFO 
2025-06-24T09:05:38.117369795Z INFO 'existing-key' in result: true
2025-06-24T09:05:38.117386855Z INFO 'non-existing-key' in result: false
2025-06-24T09:05:38.117392625Z INFO 
2025-06-24T09:05:38.117400715Z INFO secrets['existing-key']: this-is-the-secret-value
2025-06-24T09:05:38.117414705Z INFO secrets['non-existing-key']: undefined
2025-06-24T09:05:38.117422015Z INFO 
=== getSecret() behavior ===
2025-06-24T09:05:38.371347878Z INFO getSecret('existing-key'): this-is-the-secret-value
2025-06-24T09:05:38.371386039Z INFO getSecret('non-existing-key'): undefined
2025-06-24 09:05:38.371439 +0000 UTC FINISHED

[mocchira]

LGTM