rtc: measure data blob keys by their content, not their text form

[RaphaelFakhri]

Fixes data blob key validation and size accounting, which measured the text form of the key message instead of the key itself.

Problem

HandleStoreDataBlobRequest and LimitConfig compute the key length with DataBlobKey.String(). For a generic key that returns the protobuf text form, for example generic:"abc", not abc. This causes three defects:

• A key is rejected when it is within max_data_blob_key_length. With a limit of 5, the key abcde is refused because its text form is 15 bytes.
• An empty generic key is accepted. Its text form, generic:"", is not empty, so the "key is required" check never triggers.
• max_data_blobs_size counts about 10 extra bytes per blob, so participants hit the limit early.

The text form of a protobuf message is also documented as unstable, so the counted length can change between builds.

Changes

• Add config.DataBlobKeyLength, which returns the length of the generic key, or the schema name plus custom encoding for a schema ID key.
• Use it in CheckDataBlobKeyLength, CheckDataBlobsSize, CanAddDataBlob and the store handler's empty-key check.
• CheckDataBlobKeyLength now takes a *livekit.DataBlobKey instead of a string. Its only caller is the store handler.

Tests

Three new cases in TestHandleStoreDataBlobRequest: empty generic key, key exactly at the length limit, and a blob whose key plus contents exactly fits the size limit.

go test ./pkg/rtc -run TestHandleStoreDataBlobRequest -count=1 -v

Without the fix, 3 of 11 subtests fail. With the fix, all 11 pass. go vet ./pkg/config ./pkg/rtc and go test ./pkg/config pass.

[CLAassistant]

All committers have signed the CLA.

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.

[boks1971]

Thank you very much for these submissions.

But, please note that these cannot be merged without you signing the CLA.

[RaphaelFakhri]

Much obliged! I have signed accordingly.