Improve ML-DSA and SLH-DSA steps for the supports() method

[panva]

The SubtleCrypto.supports() method determines support by executing an operation's steps until it encounters unavailable key or data, key generation, a return, or an exception.

The ML-DSA sign and verify operations and the SLH-DSA sign operation currently reach an unavailable key before the 255-byte context limit can be observed. This makes supports() return true for a context that cannot be accepted with any key or message.

This PR makes those unconditional failures observable before supports() reaches its stopping point. It is the Modern Algorithms counterpart to w3c/webcrypto#558.

Changes

• Check the 255-byte context limit before key access in ML-DSA sign and verify operations.
• Check the 255-byte context limit before key access in the SLH-DSA sign operation.
• Leave SLH-DSA verification unchanged because FIPS 205 returns false, rather than an error, for an oversized context.

Successful operations are unchanged.

Examples

Each of the following now returns false.

ML-DSA context

SubtleCrypto.supports("sign", {
  name: "ML-DSA-44",
  context: new Uint8Array(256),
})

No ML-DSA key or message can make a 256-byte context valid.

SLH-DSA signing context

SubtleCrypto.supports("sign", {
  name: "SLH-DSA-SHA2-128s",
  context: new Uint8Array(256),
})

No SLH-DSA key or message can make a 256-byte signing context valid. Verification remains unchanged because FIPS 205 returns false rather than an error for an oversized context.

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