fix: allow string values in customFields for text and date fields

[Flupinochan]

Summary

Allow string values in customFields for text and date field types.

Problem

The value field in customFields only accepted number | number[],
making it impossible to set values for text or date custom fields,
which require string values according to the Backlog API specification.

Changes

• src/tools/addIssue.ts
  • Expanded value schema to accept string | number | string[] | number[]
  • Updated the MCP tool field description for value to document all supported value types
• src/tools/updateIssue.ts
  • Applied the same changes

Test

• Manually verified on a local Docker container via the MCP tool that creating and updating issues with both string (date-type) and numeric custom field values work correctly
• No unit tests were added because the underlying logic already handles both string and number values identically, and all existing tests pass

Closes #118

[katayama8000]

@Flupinochan thanks for the contribution.

[Flupinochan]

@katayama8000
Thank you for the review and merging it! I'm glad I could contribute.