mcp: support per-resource cache policy for resources/read

[CocaineCong]

Fixes #1185

Desc: support per-resource cache policy for resources/read

Changes:

• Add exported CacheHint{ TTLMs int, CacheScope string } type.
• Add a CacheHint *CacheHint field to Resource and ResourceTemplate, tagged json:"-" so it never appears in resources/list or resources/templates/list.
• readResource now applies the matched resource's/template's hint to the resources/read result via a new (*Cacheable).applyCacheHint.
• Add UnitTest and passed.
• Update docs/server.md.