docs: add XML documentation for modern asymmetric signers and X-DH agreement (Batch 11)

[KonradSop]

Adds XML documentation to the nine streaming primitives that pair with the modern asymmetric parameter classes covered by PRs #673 (Batch 5), #677 (Batch 7), #678 (Batch 8), #679 (Batch 9) and the open Batch 10 generators PR. With this batch the ISigner / IRawAgreement surface for ML-DSA, SLH-DSA, EdDSA and X-DH now carries the same depth of documentation as the parameters and key-pair generators.

• MLDsaSigner — class summary citing FIPS 204; Init documents the ParametersWithContext (≤255 bytes) and ParametersWithRandom envelope unwrapping plus the parameter-set / key-set match check; GenerateSignature, VerifySignature, Reset documented with their full exception contracts.
• SlhDsaSigner — same shape as ML-DSA, citing FIPS 205; class summary explains the buffered context-prefixed message pattern.
• Ed25519Signer / Ed25519ctxSigner / Ed25519phSigner — class summaries citing RFC 8032 §5.1; Ed25519ctxSigner and Ed25519phSigner constructors document the cloned context parameter and the ArgumentNullException raised on a null context; the pure-Ed25519 signer explicitly notes that no context is permitted.
• Ed448Signer / Ed448phSigner — same shape as the Ed25519 ctx/ph signers, citing RFC 8032; class summaries note the SHAKE256 pre-hash for the ph variant.
• X25519Agreement / X448Agreement — class summaries citing RFC 7748; Init documents the cast contract; CalculateAgreement documents the InvalidOperationException raised when the agreement produces an all-zero secret (degenerate peer key).

Key Accomplishments

• Closes the modern-asymmetric documentation surface: with KeyGenerationParameters (Batch 9), key-pair generators (Batch 10) and now the signer / agreement primitives documented, every user-facing class in the ML-DSA / SLH-DSA / EdDSA / X-DH stack has consistent XML coverage.
• Accurate exception contracts: <exception> tags only added where the method body actually throws — InvalidCastException on Init casts, InvalidOperationException on direction mismatches and pre-hash finalisation failures, ArgumentOutOfRangeException when the context exceeds 255 bytes for the PQC signers, ArgumentException on parameter-set mismatch.
• Fixed two pre-existing CS1573 warnings: added the missing <param name="parameters"> tags on MLDsaSigner and SlhDsaSigner constructors that already documented deterministic.
• Consistent style: Class summaries follow the same FIPS / RFC reference pattern used in Batches 5/7/8/9/10. Internal members and pre-existing implementation comments left untouched.
• No overlap with open PRs: scope is disjoint from feature/modern-curves-docs (docs: add XML documentation for modern-curve parameters (Batch 7) #677), feature/slh-dsa-docs (docs: add XML documentation for SLH-DSA parameters (Batch 8) #678), feature/ml-pqc-gaps-docs (docs: add XML documentation for ML-KEM and ML-DSA parameter gaps (Batch 9) #679) and feature/keypair-generators-docs (Batch 10).

Verification

• Build Status: dotnet build crypto/src/BouncyCastle.Crypto.csproj -c Release — succeeded on net6.0, netstandard2.0, net461 with no new warnings. The two CS1573 warnings introduced by the first pass were resolved before push.
• Scope: Documentation-only; no behavioural or signature changes.

Checklist before requesting a review

• I have performed a self-review of my code
• I have kept the patch limited to only change the parts related to the patch
• This change requires a documentation update

See also Contributing Guidelines.