Fix SetEnv SSH config parsing and accumulation

[EhabY]

• Add parseSshConfig function to correctly parse SetEnv MY_VAR=value (previously split on first = producing incorrect key/value)
• Modify mergeSshConfigValues to concatenate SetEnv values instead of replacing, preserving the default CODER_SSH_SESSION_TYPE
• Ignore empty SetEnv values to prevent accidentally clearing defaults

Closes #559

[deansheather]

Are there any other ssh options that can be specified multiple times? Would be nice to support those as well

[EhabY]

Are there any other ssh options that can be specified multiple times? Would be nice to support those as well

I had Claude look into the ssh_config documentation and here is the full list. We only care about multiple entries so in this case, it would be IdentityFile, CertificateFile, SetEnv, SendEnv, LocalForward, RemoteForward, DynamicForward can be accumulating. Alternatively, why not let the SSH client handle it? Like we can just pass the configs as is (override configs would be the put first since it's "first match wins"). I do not love that we have to re-implement this override behavior ourselves 🤔. If we want leaner files then we can still apply the overriding but we can always add entries from the list above as is (we do not manually combine them but pass them as is, such that override is earlier than the base config).

Claude's Analysis

SSH Client (ssh_config) Accumulating Options

Options that accumulate across multiple declarations, unlike standard "first match wins" behavior.

Option	Example	Notes
IdentityFile	IdentityFile ~/.ssh/id_ed25519 IdentityFile ~/.ssh/id_rsa	Tried in sequence
CertificateFile	CertificateFile ~/.ssh/cert1.pub CertificateFile ~/.ssh/cert2.pub	Tried in sequence
SetEnv	SetEnv FOO=bar SetEnv BAZ=qux	Both sent to server
SendEnv	SendEnv LANG LC_* SendEnv MY_VAR	Prefix with - to clear
LocalForward	LocalForward 8080 localhost:80 LocalForward 9090 localhost:90	Multiple tunnels
RemoteForward	RemoteForward 8080 localhost:80	Multiple tunnels
DynamicForward	DynamicForward 1080 DynamicForward 1081	Multiple SOCKS proxies
GlobalKnownHostsFile	GlobalKnownHostsFile /etc/ssh/known_hosts /etc/ssh/known_hosts2	Space-separated
UserKnownHostsFile	UserKnownHostsFile ~/.ssh/known_hosts ~/.ssh/known_hosts2	Space-separated
PermitRemoteOpen	PermitRemoteOpen host1:80 host2:443	Space-separated
CanonicalDomains	CanonicalDomains example.com corp.local	Space-separated

Source

OpenBSD ssh_config(5) — the canonical reference (OpenSSH is developed by OpenBSD).