feat: implement base64-aware size validation for building_block_definition symbol

[nroi]

BD-2290
BD-2290

related PR: https://github.com/meshcloud/meshfed-release/pull/9620

I've tested it locally. A tofu plan goes through with an image that is exactly 100 KiB of size, like so:

symbol       = "data:image/svg+xml;base64,${filebase64("/path/to/svg/exactly-100kib.svg")}"

with an image that's 100 KiB plus one byte, we get this error during plan:

╷
│ Error: Symbol Image Too Large
│
│   with meshstack_building_block_definition.example_03_manual,
│   on main.tf line 27, in resource "meshstack_building_block_definition" "example_03_manual":
│   27: resource "meshstack_building_block_definition" "example_03_manual" {
│
│ The decoded symbol image is 102401 bytes, which exceeds the maximum allowed size of 100 KiB (102400 bytes).
╵

[Copilot]

Pull request overview

Implements client-side validation for meshstack_building_block_definition.spec.symbol to ensure image data URIs are base64-valid and within the decoded size limit, improving early feedback in Terraform plans/applies.

Changes:

• Added a SymbolSize string validator that accepts http(s):// URLs or whitelisted data:image/*;base64, URIs and enforces a 100 KiB decoded limit.
• Wired the new validator into the Building Block Definition resource schema for spec.symbol.
• Added provider tests covering valid/invalid URLs, MIME types, base64 correctness, and boundary size conditions.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
internal/validators/symbol_size_validator.go	New string validator implementing base64-aware symbol validation and decoded-size enforcement.
internal/provider/building_block_definition_resource_schema.go	Applies validators.SymbolSize{} to spec.symbol schema attribute.
internal/provider/building_block_definition_resource_test.go	Adds test matrix for symbol validation behavior via resource test steps.